Biology, Sixth Edition
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Chapter 10, The Basic Principles of Heredity
The Basic Principles of Heredity “transmission of genetic information from parent to offspring”
Topic 3 & 8
Biology, Sixth Edition
Copyright 2002 by Harcourt College Publishers, a division of Thomson Learning
Chapter 10, The Basic Principles of Heredity
Lecture Topics
• Mendel and the process of science • Mendel‘s principles of inheritance
• Unit characteristics, segregation, independent assortment and dominance • Product and sum laws
• • • •
Crosses: Parental, F1, F2, and test Chromosome mapping Variations on Mendelian inheritance In-breeding and out-breeding
Biology, Sixth Edition
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Chapter 10, The Basic Principles of Heredity
Why Study Mendel? (3.3)
• Mendel was the first to demonstrate the principles of inheritance • He used a systematic scheme based upon an understanding of mathematics and statistics before formal statistics was developed • He applied his knowledge of mathematics with an excellent scientific method
Biology, Sixth Edition
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Chapter 10, The Basic Principles of Heredity
Mendel Disproved Blending
• The age-old way of thinking of inheritance was the concept of the blending of characters (characteristics) • We look somewhat like our parents • Breeders would combine desired characteristics in crosses of domestic animals • The hope (and thinking) was that characteristics were merely added together – blending – in the new generation. • But, blending of characters is incorrect: • After 1000's of generations, we ought to be virtually identical! Yet, neither domestic animals nor ourselves all look the same. • We resemble our parents, but are clearly distinct from them • No one studied the problem systematically until Gregor Mendel (1822-1884)
Biology, Sixth Edition
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Chapter 10, The Basic Principles of Heredity
•
•
•
•
Breeders knew of hybrids at the time that Mendel started his experiments (in 1856), but only knew that progeny resembled each parent, and that some resembled grandparents. No one knew why. Mendel developed a systematic way of understanding heredity, using very good scientific technique. He applied the scientific method combined with mathematical analysis to demonstrate: 1. Unit characteristics 2. Segregation of those characteristics 3. Dominance of some characteristics 4. Independent assortment of characteristics Today, we substitute the term ―gene‖ for characteristics
What Did Mendel Show?
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Chapter 10, The Basic Principles of Heredity
Mendel Carefully Chose His Organism
•
Pisum sativum: the garden pea
1. Very productive: produces many peas (large N: good statistics) 2. Short life cycle: produce many generations in a short time 3. Typically self-pollinating: good for inbreeding 4. Easily cross-pollinated due to flower structure Has 7 distinct phenotypic characteristics: 1. Yellow versus green seeds 2. Round versus wrinkled seeds 3. Green versus yellow pods 4. Tall versus short plants 5. Fat versus tight pods 6. White versus grey seed coats 7. Flowers: end of stem versus along the length of stem
•
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Chapter 10, The Basic Principles of Heredity
The Pea Flower
• The flower is well-suited to this study, which required controlled pollination • Protects the reproductive apparatus • Male (anther: makes pollen) • Female (ovary: makes ova) • Stigma arises from the ovary • Good for inbreeding and artificial breeding • Can bag flowers, easily cut off the anthers to prevent further pollination
Biology, Sixth Edition
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Chapter 10, The Basic Principles of Heredity
True-Breeding Stocks and Terms
• Mendel needed stocks of plants that bred in a reliable manner • Only way to be sure that they are reliable, initially, is to develop true-breeding stock • True-breeding stock produce progeny that are just like the parent with regard to any & all characteristics mentioned previously. • Mendel achieved true-breeding stocks by repeated inbreeding for several years • Inbreeding means to breed parent to progeny (mother with son and/or father with daughter) • He inbred his stocks many times to produce truebreeding stocks.
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Chapter 10, The Basic Principles of Heredity
Put Another Way...
• When parents were bred to the same true breeding type (e.g. Tall), they always bred true (i.e. their progeny were tall; so were their grandchildren and their great grandchildren.) So, P X P of the same type, yielded P phenotype. • In other words, parental type crossed into
parental type yields parental type • Such stocks are true-breeding
Biology, Sixth Edition
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Chapter 10, The Basic Principles of Heredity
Selected Crosses Produce Specific Progeny Types
• Definitions. We use specific symbolic notation to represent specific crosses: • P represents the parental generation of true
breeders • F1 represents the the 1st filial generation – the progeny of crossing true-breeding P generation individuals that had varied in one character. • F2 represents the the 2nd filial generation, derived from crossing the F1 generation to itself at random
Biology, Sixth Edition
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Chapter 10, The Basic Principles of Heredity
One of Mendel‟s Crosses
• Mendel knew nothing of the chemical basis for inheritance He had to work very carefully and examine ONLY the outward appearance of the plants He compared what we call today the phenotype, which is the outward expression of the genes
•
•
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Chapter 10, The Basic Principles of Heredity
A Parental Cross
• Homozygous Tall (TT) X Homozygous Short (tt)
• •
All of the F1 generation are tall.
Demonstrates the concept of dominance genotype
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Chapter 10, The Basic Principles of Heredity
When Ps of Different Type Were Bred… • If tall true-breeding parents were crossed with short true-breeding parents, the offspring were tall • But if those progeny were crossed into themselves (i.e. if that population interbred), then the original parental types reappeared • This was an unexpected breakthrough!
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Chapter 10, The Basic Principles of Heredity
•
Cross the F1 generation at random to itself.
Cross the F1s
•
•
1/4 of F2 (the progeny of the F1 cross) are short
Mendel showed that the short plant bred true, so it was a parental type. That a parental phenotype disappeared and then reappeared destroys the concept of blending. • It was masked in the F1 Instead, it supports the
•
•
concept of unit characteristics – genes, as we now understand them
Biology, Sixth Edition
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Chapter 10, The Basic Principles of Heredity
Segregation of Characteristics (3.2.6-7)
• Mendel recognized that the only way to achieve the masking of the characteristics was to segregate (separate) them somehow in the gametes. • The characters must have been separated during formation of the pollen (male) and the ova (female). • This is the concept of segregation.
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Chapter 10, The Basic Principles of Heredity
Chromosomal Basis of Segregation • Segregation occurs in meiosis at MI and then at MII • As a result, the ova or sperm contribute different homologous chromosomes to the progeny
Ova or sperm
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Chapter 10, The Basic Principles of Heredity
Mendel‟s Concepts & Modern Terminology
• Mendel worked in a strictly conceptual framework — he had NO IDEA of the nature of the genetic material • Today we know his ‗characters‘ are the products of genes. The collection of genes is the genotype, or genome. • A gene is positioned at a given locus (loci, plural). • Position is very important, it can define the effect of a gene • Regulation of genes is position-dependent.
Biology, Sixth Edition
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Chapter 10, The Basic Principles of Heredity
Alleles
• Different varieties of genes are called alleles. • Since diploid (2n) organisms have 2 sets of chromosomes, there are two possible alleles for each gene locus in a 2n organism. • If those alleles are the same, the organism is homozygous at that location • If different, it is heterozygous
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Chapter 10, The Basic Principles of Heredity
Homozygotes & Heterozygotes
• We indicate the genotype by the letters: • Dominant alleles are represented by capital letters • Recessive alleles are represented by lower case letters • Examples: • TT is homozygous dominant • Phenotype is dominant • tt is homozygous recessive • Phenotype is recessive • Tt is heterozygous • Phenotype is dominant
Biology, Sixth Edition
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Chapter 10, The Basic Principles of Heredity
Homologs & Alleles
A B a b
• 2n organisms have a chromosome received from each parent. • Called homologous chromosomes • These have alleles of A, B, C & D. • A & a, B & b, C & c, D & d • C is NOT an allele to D; nor is A or B • A locus is a physical location of DNA that encodes for a protein product; i.e. a gene • A, B, C & D are at specific loci
Homologs
C
From Dad
c
D
From Mom
d
Biology, Sixth Edition
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Chapter 10, The Basic Principles of Heredity
Diploid cells
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Chapter 10, The Basic Principles of Heredity
• (3.2.2)
Homologous and Nonhomologous Chromosomes: Definitions
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Chapter 10, The Basic Principles of Heredity
Homologs in One Cell
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Chapter 10, The Basic Principles of Heredity
During Fertilization, Homologs From Each Parent Come Together
Biology, Sixth Edition
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Chapter 10, The Basic Principles of Heredity
Punnet Squares (3.3.2)
• What tools can we use to predict each generation? • Simplest tool is a Punnet Square
• Consider:
• Aa (male) X Aa (female) • Is an internal F1 cross • Place one set of alleles from one of parents on top; the other set from the other parent on the side. Location doesn‘t matter.
A
A
a
a
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Chapter 10, The Basic Principles of Heredity
Calculating Punnet Squares
• Determine the allelic Parents composition of the progeny (blue) merely by adding the alleles together (red arrows in two examples). • You should have two alleles for each progeny (recall they are 2n)
A A
a AA
a Aa aa
Aa
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Chapter 10, The Basic Principles of Heredity
A Parental Cross Produces the F1
• • • • Cross: Two different true-breeding strains: (Male: BB) X (Female: bb) All progeny have the same genotype (Bb) All have a dominant phenotype (B)
Biology, Sixth Edition
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Chapter 10, The Basic Principles of Heredity
Guinea Pigs Follow Mendel‟s Laws
• True-breeding guinea pigs of different color • Produce F1 of only one color: dominant black
B b b B
Bb Bb
Bb Bb
F1
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Chapter 10, The Basic Principles of Heredity
Cross the F1 Generation
• Guinea pigs will display typical F1 and F2 inheritance • 3 of 4 babies will be black; one will be brown • Of those three, one will be true-breeding; the others are not
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Chapter 10, The Basic Principles of Heredity
F1 Breeding Results
• Produces the F2 generation: • 3:1 phenotype — 3 dominant (bold below; BB, Bb): 1 recessive • 1:2:1 genotype — 1 homozygous dominant (BB), 2 heterozygous (Bb), 1 homozygous recessive (bb) • 3:1 phenotypic and 1:2:1 genotypic ratios are typical of monohybrid (i.e. considering only one gene, as here) F1 crosses
B B b
BB Bb
b
Bb bb
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Chapter 10, The Basic Principles of Heredity
Practice
• http://biology.clc.uc.edu/courses/bio105/gen eprob.htm
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Chapter 10, The Basic Principles of Heredity
A Test Cross
• What if you only know that you have a black and a brown guinea pig (i.e. you don‘t know the genotype)? • Do a test cross • Cross with a known true-breeding recessive brown guinea pig
Biology, Sixth Edition
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Chapter 10, The Basic Principles of Heredity
Test Crosses: Mechanism
• On inspection, heterozygotes are indistinguishable from homozygote dominants. • Phenotypes can be tested for genotype with test crosses… • Test crosses are easily checked w/ Punnett squares. • Always cross the unknown into
Test
a A A Aa Aa a Aa Aa
All A phenotype: Homozygous A
homozygous recessive to reveal the dominant genotype.
Test
A a
a Aa aa
a Aa aa
• The distribution of alleles follows the product law and the sum law.
½ A phenotype; ½ a phenotype
Biology, Sixth Edition
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Chapter 10, The Basic Principles of Heredity
Predicting Each Generation: The Law of Sums and the Law of Products
• The frequency of independent events, such as the mixing of unlinked genes by sexual reproduction, can be described by sum and product laws. • Sums Law: The total proportion of a given genotype within a population can be determined by adding up all the allele combinations of its individuals. • Product Law: The recombination of genes is entirely random. Thus, the proportion of the individual alleles provided by the parents can be multiplied together to get the proportion of the recombined set of alleles in the next generation. • The concepts are depicted using random tosses of pennies, in the next slide:
Biology, Sixth Edition
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Chapter 10, The Basic Principles of Heredity
The Penny Solution
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Chapter 10, The Basic Principles of Heredity
The Rules of Probabilities
• The probability of a penny falling face up is 0.5; tails up, 0.5 • What is the probability of all possible combinations of heads and tails in two tosses? • Recombining the probabilities of a head following a head is given by multiplying their probabilities:
• Head then head, 0.5 X 0.5 • Tail then head, 0.5 X 0.5 (do this twice) • Tail then tail, 0.5 X 0.5
• To find out what the total probabilities of all of the different combinations is, sum alike outcomes. • Note that the possibility of a combination of head and tail occurs twice as often as any other category.
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Chapter 10, The Basic Principles of Heredity
A Recap of the Law of Sums and the Law of Products
• The frequency of independent events, such as the mixing of unlinked genes can be described by sum and product laws • The Punnet Square takes advantage of the observation that
gene frequencies are the result of simple sums and products • Sums: because the proportion of a given genotype of a population can be determined by adding up all the gene combinations of its individuals.
Because the additions occur irrespective of the makeup of each parent, the total summed quantity of each genotype is independent of the previous proportions. • Products: because the proportion of any given genotype in a
population can be predicted from the product of the proportion of those allele frequencies in the parents‘ donated gametes.
Thus, the proportion of each genotype and allele in each generation is dependent upon the proportions of the previous generation.
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Chapter 10, The Basic Principles of Heredity
Independent Assortment (8.1.5-6)
• When Mendel crossed strains that had two different sets of characteristics, those characteristics behaved as if the alternate phenotype didn‘t exist. i.e. — • They assorted independently in the mating. • They apparently weren‘t physically connected to each other • In other words, they were nonlinked characters (we say today that they are nonlinked genes) • Mendel was either lucky or extra cautious, or both: • We know today that some genes are actually linked • But Mendel‘s idea is still a generally correct concept.
• (many genes are NOT linked)
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Chapter 10, The Basic Principles of Heredity
Dihybrid Crosses (8.2.2)
• When two individuals mate, they recombine MANY genes. • If we consider what happens to two genes at different loci at the same time, we are describing a dihybrid cross • If three, a trihybrid cross. • In each case, as long as the genes are nonlinked, Mendel‘s laws hold, and the sum and product laws hold. • Thus, a dihybrid cross is merely an extension of what happens in two parallel and unrelated monohybrid crosses; i.e. not only do alleles of the same gene mix together in an unbiased manner, different genes will mix together randomly. • If the dihybrid cross occurs this way, then it is strong evidence for independent assortment (i.e. the genes recombine without influence from each other).
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The Typical Ratios of a Dihybrid Cross: AaBb X AaBb
AB Ab AABb AAbb AaBb
Aabb aB AaBB AaBb
Chapter 10, The Basic Principles of Heredity
ab AaBb Aabb aaBb aabb
AB
Ab
AABB AABb
aB ab
AaBB AaBb
aaBB aaBb
•Out of a total of 16 different possibilities: •Phenotypic ratio —9:3:3:1 9 Dbl Dom: 3 ½ Dom: an alternate 3 ½ Dom: 1, Dbl Recessive •Genotypic ratio —1:1:2:2:4:2:2:1:1 1 AABB:1AAbb:2AABb:2AaBB:4AaBb:2aaBb:2 Aabb:1aaBB:1aabb
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Chapter 10, The Basic Principles of Heredity
Dihybrid Crosses With Guinea Pigs
• The first cross, considering two genes in true-breeding parents, has predictable outcome • All dominant in the F1
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Chapter 10, The Basic Principles of Heredity
Crossing the F1
• There are 4 different possible gametes from each parent. • These cross to form 16 (= 42) possible genotypes in the F2. • The dihybrid F2 phenotypic ratio is 9:3:3:1 • But the genotypic ratio is 1:1:2:2:4:2:2:1:1 • All of this holds true if the genes are unlinked.
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Chapter 10, The Basic Principles of Heredity
Understanding the Genotype
• Mendel‘s Laws hold today. • But, we know that some genes are linked
• i.e. they are on the same piece of DNA
• How do we understand the genetics of linked genes? Mendel‘s Laws still hold, but with modification and further insight • The key is to understand independent assortment and the mechanisms of genetic recombination
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Meiosis Mechanisms Produce Independent Assortment
Chapter 10, The Basic Principles of Heredity
• Meiosis I provides the independent assortment of chromosomes and genes into the gametes. • The homologs are arranged at random at the equatorial plane in Metaphase I • This results in four different possible combinations of chromosomes and genes in the gametes, in a 1:1:1:1 ratio:
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(8.1.4; 8.2.2) Recombination by Crossover Mixes Up Genes
Chapter 10, The Basic Principles of Heredity
• In prophase I, crossover can occur • This results in recombination (different from parents) of genetic information between the homologous pairs • If crossover occurs between two chromatids of the tetrad, two of
the four gametes produced will have a recombinant set of chromosomes
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Chapter 10, The Basic Principles of Heredity
A Double Crossover Produces Nonrecombinant Alleles
• If crossover occurs twice between two genes, the alleles will not be recombinant
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Chapter 10, The Basic Principles of Heredity
Linkage (8.3.4-5)
• Simply put, when one allele moves during recombination and another moves with it, then we know that... • Such genes are linked (linkage group (8.3.3); i.e they are on the same physical chromosome – the same piece of DNA. • Linked genes do not show independent assortment, and so interfere with Mendelian frequencies. • Linked genes can therefore be easily detected: • If the frequency distribution of genes in the
progeny in a dihybrid cross isn‘t Mendelian, the genes are linked. How can we tell if this is the case?
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Chapter 10, The Basic Principles of Heredity
A Case Test for Linkage
• We test for linkage using a two-point test cross (2-point refers to using two different loci to test for linkage) • Is wing size and color linked in Drosophila? • B produces grey color; b: black • V: normal wings; v: vestigial • Make a dihybrid, 2-point test cross between truebreeding males homozygous for two recessive genes and true-breeding females homozygous dominant for those genes
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Chapter 10, The Basic Principles of Heredity
The Two-point Test Cross
• BV X bv will produce a F1 with phenotype of the entire population appearing as dominant in both characteristics – it is dihybrid. • If the F1 is dihybrid and if the genes are not linked, the F1 will produce 4 different potential gamete types: BW, Bw, bW, and bw in equal numbers. • Thus, in the absence of linkage, there will be a 1:1:1:1 ratio between these gamete genotypes • Each has the same probability of recombining with the opposite sex‘s genes.
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Chapter 10, The Basic Principles of Heredity
Two-Point Test Cross, Continued
• If the B(b) and W(w) loci are linked, they will not assort in a Mendelian manner in the F2 • BW and Bw have the same (dominant) phenotype in B; BW and bW have the same phenotype in W. • Recall that in order to test for the cause of a dominant phenotype, we cross that phenotype into homozygous recessive and look at the ratio of the phenotypes of the next generation.
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Chapter 10, The Basic Principles of Heredity
Two-Point Test Cross I
• The parental gametes are bv (male) and BV (female) • As expected, they produce a normal F1
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Chapter 10, The Basic Principles of Heredity
Two-Point Test Cross II
• Now cross the F1 (BbVv) to a homozygous recessive (male, black with vestigial wings) • You expect a 1:1:1:1 phenotypic ratio in the progeny – if B and W are unlinked. • Instead, We observe a
4:4:1:1 phenotypic ratio. • This overabundance can occur only if B and W are linked. • When B and W are linked, Bv & bV gametes that give rise to alternate phenotypes can only be produced by meiotic crossover.
• Usually show linkage as
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Biology, Sixth Edition
Chapter 10, The Basic Principles of Heredity
• vertical pairs: (8.3.4-5) TB ___ instead of TtBb tb lines are taken to represent the chromosomes indicating linkage visually. Linked alleles and cross-over allele combos are clear. Test cross of above yields recombinants of Tb tB
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Chapter 10, The Basic Principles of Heredity
Crossover As a Tool for Determining the Relative Location of Genetic Loci
• Crossover is assumed to occur with equal frequency
for each location along the length of a chromosome.
• If there are many crossovers between 2 loci, they are rather far apart. • In contrast, if two loci show little crossover they are close together. • In the previous example, 20% of the gametes must have undergone crossover (rightmost two genotypes in the Punnett Square – 10% + 10% = 20%). • The distances between loci are related to the rate of crossover – one percent = one map unit. • In the case above, crossover occurs 20% of the time so 20 map units separate the B and W loci.
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Chapter 10, The Basic Principles of Heredity
Mapping Chromosomes
• If there is 8% crossover between A & C, 5% crossover between A & B, and 3% between B and C, we can arrange them in this manner (a): • If the crossover between A and C was 2% instead of 8%, we could rearrange as in (b):
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Chapter 10, The Basic Principles of Heredity
Sex Chromosomes (3.3.6-7;8.3.1)
• The chromosomes that determine individual‘s sex are called the sex chromosomes. • Usually, the sex chromosomes are X and Y. • Usually one sex is homogametic, having two of the same chromosome type; the other is heterogametic, have two different sex chromosomes. • In mammals, females are homogametic and are XX;
males are XY and are heterogametic • Other chromosomes are called the autosomes.
Humans have 22 autosomes and females have a pair of X chromosomes; males have an X and a Y
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Chapter 10, The Basic Principles of Heredity
The Sperm Decides the Sex of the Progeny
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Chapter 10, The Basic Principles of Heredity
Sex-Linkage (3.3.8-12)
• Sex-linked traits are defined by genes on the the X chromosome. (Until recently it was thought Y had no genes. There are a few, but we will ignore them here.) • Because sex-linkage really refers to X-linkage, females can be
heterozygous for sex-linked traits
• Can mask the presence of a recessive gene. • Often recessives are undesired. • Hence, the ominous-sounding term ―carrier‖ female. • In contrast, males directly express whatever alleles are on their X chromosomes. Both dominant or recessive sex-linked traits are expressed, because males are heterogametic, or hemizygous, having only one X chromosome.
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Sex Linkage
Chapter 10, The Basic Principles of Heredity
Heterozygous Female X X
Hemizygous Male X Y
A
a
A
No ‘a’ locus
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Chapter 10, The Basic Principles of Heredity
Hermaphrodites
• Some organisms contain the both types of sex organs in one individual; they are called hermaphrodites. • Hermaphrodites do not have sex chromosomes. • Many flowering plants are hermaphrodites – they are monecious, having different flower types on the same plant. Peas are a good example. • Some are dioecious, with different sexes in different plants – gingkos are one example. • Some animals are hermaphrodites – this is not uncommon in invertebrates (organisms without a backbone). • Some carry both sexes at the same time (e.g. ctenophores) • Some change sex over time (e.g. some molluscs such as Crepidula)
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Chapter 10, The Basic Principles of Heredity
Sex-Linked (X-linked) Traits
• Such traits are controlled by X-linked genes
• Hemophilia Xh • Color blindness Xb
• Some sex-associated traits are autosomal
• Male-pattern baldness
• X-linked color blindness is depicted in the next slide:
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Chapter 10, The Basic Principles of Heredity
Color Blindness
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Chapter 10, The Basic Principles of Heredity
Drosophia Handouts/Lab
Fly Cycle Video
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Chapter 10, The Basic Principles of Heredity
Dosage Compensation
• Females would have 2X the dosage of gene product (called gene dosage) from the Xs, compared to the Y chromosome, if not for dosage compensation: • In fruit flies males compensate by more actively reading the X chromosome. • In mammals one of the X chromosomes is turned off at random. • The inactive X is greatly compacted and appears as a darkly-stained dot in female nuclei. The dot is called the Barr Body. (Cellular Gender LAB
activity)
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Chapter 10, The Basic Principles of Heredity
Human Female Blood smear “drumstick”
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Barr Body
Chapter 10, The Basic Principles of Heredity
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Chapter 10, The Basic Principles of Heredity
Human Male Chromosome
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Chapter 10, The Basic Principles of Heredity
Human Female Chromosome
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Chapter 10, The Basic Principles of Heredity
X-Inactivation Effects
• X-inactivation occurs during development in the early progenitor cells for organ systems and persists for the life of the cell. Sometime X-inactivation is clearly evident; sometimes not. It is rarely detrimental. It is generally referred to as variegation. • Examples: • In human females, sweat glands are sometimes heterogeneously distributed, causing a nondangerous condition termed anhydrotic ectodermal dysplasia – in skin containing inactive X chromosomes that bear the only gene for sweat gland function, such women do not perspire. • Color vision can be affected – red/green perception • Hemophilia can exhibit dosage compensation • Cat coloration is X-linked – calico cats are females
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Chapter 10, The Basic Principles of Heredity
Calico Cat: X-inactivation
Biology, Sixth Edition
Copyright 2002 by Harcourt College Publishers, a division of Thomson Learning
Chapter 10, The Basic Principles of Heredity
The Path From Genotype to Phenotype Can Be Complex
Genotype Phenotype
DNA
RNA
(Central Dogma)
Protein Tissue
Cell
At any stage, there can be feedback on the production of protein, RNA or DNA
Metabolism Organism
Organ
Population Behavior
Biology, Sixth Edition
Copyright 2002 by Harcourt College Publishers, a division of Thomson Learning
Chapter 10, The Basic Principles of Heredity
„Non-Mendelian‟ Genetics
• Inheritance sometimes does not follow the simple concepts of simple dominance and duality of gene types pioneered by Mendel. There exist: • Incomplete dominance • Dominant character influences, but does not overwhelm, the heterozygote • Flower color: snapdragons and 4 O'clocks • Codominance (3.3.13) • Heterozygote displays characteristics of two dominant alleles • Roan horses have red AND white hair, with different pigmentation occurring in the same strand of hair • ABO blood typing: several red blood cell antigens can occur on cell surface (also multiple alleles 3.3.4-5)
Biology, Sixth Edition
Copyright 2002 by Harcourt College Publishers, a division of Thomson Learning
Chapter 10, The Basic Principles of Heredity
Incomplete Dominance
Biology, Sixth Edition
Copyright 2002 by Harcourt College Publishers, a division of Thomson Learning
Chapter 10, The Basic Principles of Heredity
„Non-Mendelian‟ Genetics, Continued
• Multiple alleles may control a trait • Polygenic inheritance (8.4.1-2) refers to the control of one apparent trait by more than one gene • Skin color(melanin production) is controlled by more than 2 genes; probably 3-4. • Many genes display pleiotropy, in which one gene can affect many different characteristics
• A gene displays epistasis if it can mask or prevent expression of another gene or genes (Koala albinism) • A homozygous recessive gene can prevent expression of another gene; a dominant heterozygote can have the same effect.
Biology, Sixth Edition
Copyright 2002 by Harcourt College Publishers, a division of Thomson Learning
Chapter 10, The Basic Principles of Heredity
Polygenic Inheritance
• In humans, 6 skin color genes contribute their products to produce a gradual gradation of skin color from very pale to nearly bluish-black. • Their effects can be thought of as additive. • At first sight, this looks very much like blending. It isn‘t, however, because parental genotypes and corresponding phenotypes can be recovered.
Biology, Sixth Edition
Copyright 2002 by Harcourt College Publishers, a division of Thomson Learning
Chapter 10, The Basic Principles of Heredity
Skin Color Distribution
• Follows expected frequency distribution for a Mendelian trait • Each gene product is additive to other gene products
• Height is also polygenic, and populations show similar distributions with respect to height.
Biology, Sixth Edition
Copyright 2002 by Harcourt College Publishers, a division of Thomson Learning
Chapter 10, The Basic Principles of Heredity
Polygenic Traits
• >10 loci are involved in height inheritance in humans • As expected, the distribution of height follows a normal distribution
Biology, Sixth Edition
Copyright 2002 by Harcourt College Publishers, a division of Thomson Learning
Chapter 10, The Basic Principles of Heredity
Penetrance and Expressivity
• In addition, genes display variation in their degree of expression. Usually this arises because of gene
interaction
• The term that describes the expressivity of a gene is
penetrance
• Penetrance is expressed as the percent of a population that expresses a trait • If a gene is expressed in 100% of a population, it has 100% penetrance • If 50% of population expresses, has 50% penetrance • Demonstrates that gene expression varies from individual to individual
Biology, Sixth Edition
Copyright 2002 by Harcourt College Publishers, a division of Thomson Learning
Chapter 10, The Basic Principles of Heredity
Rooster Combs and Gene Interaction
• In roosters, 2 genes control comb structure. • The P (pea) gene controls the shape of the comb (‘single‘ vs ‗pea‘) and the R (rose) gene controls the buffont-style Rose comb. • When P and R are both present as dominants, a different type of comb is observed: Walnut. • This occurs because of gene interaction!
Copyright 2002 by Harcourt College Publishers, a division of Thomson Learning
Biology, Sixth Edition
Chapter 10, The Basic Principles of Heredity
Biology, Sixth Edition
Copyright 2002 by Harcourt College Publishers, a division of Thomson Learning
Chapter 10, The Basic Principles of Heredity
Inbreeding
• Inbreeding was useful to Mendel for producing true-breeders • But it can concentrate undesirable attributes, which are often recessive. • Human inbreeding tends to increase the frequency of rare genetic disorders, although the Tamil of India extensively intermarry with little or no ill effect. • Australian sheepdogs must be euthanized if they are white puppies: they become blind, deaf, and have many skeletal muscle problems • The English bulldog has significant bone structure problems – many members of this breed can barely mate and bear puppies, and many have difficulty breathing
Biology, Sixth Edition
Copyright 2002 by Harcourt College Publishers, a division of Thomson Learning
Chapter 10, The Basic Principles of Heredity
Outbreeding
•
Outbreeding means breeding of different strains or stocks of the
•
•
•
same organism Breeding to different strains tends to produce more robust individuals, referred to as hybrid vigor • Produces individuals that are multiply heterozygous at many loci • Masks recessives in many different traits In many cases, multiple heterozygosity seems to confer additional advantage beyond the masking of recessive traits. This is referred to as overdominance or the Heterozygote Advantage. • Most modern grain crops are multiple hybrids: they are more resistant to disease, tolerate changes in weather, etc. A simple example: Sickle Cell Anemia • People heterozygous for sickle-cell anemia have a distinct advantage in Africa, where the trait improves resistance to malaria by interfering with the parasite life cycle. • The moderate illness due to the heterozygote sickle cell condition is not severely disabling.