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					Genetics concepts: Mendel and the gene idea

Chapter 7

Gregor Mendel used the garden pea plant Pisum sativum in his studies because there were many varieties that were easily obtainable He also used peas because they can either self-pollinate or cross-pollinate, which allowed him to create purebreeding lines as well as hybrids of two different lines

The pea plants that Mendel used differed in their character forms: For instance, flower color is a character, and Mendel’s plants had either purple or white flowers Contrasting phenotypes for a particular character are the starting point for any genetic analysis

Other plant breeders had obtained results similar to those of Mendel, but he did something that, more than anything else, marks the birth of modern genetics:

He counted the numbers of plants with each phenotype

Mendel’s application of careful statistical analysis led to an explanation of the F2 ratios he observed in his experiments

Parental phenotype

F1

F2

F2 ratio

1. Round×wrinkled seeds 2. Yellow×green seeds 3. Purple×white petals 4. Inflated×pinched pods 5. Green×yellow pods

All round All yellow All purple All inflated All green

5474 round; 1850 wrinkled 6022 yellow; 2001 green 705 purple; 224 white 882 inflated; 299 pinched 428 green; 152 yellow

2.96:1 3.01:1 3.15:1 2.95:1 2.82:1

6. Axial×terminal flowers
7. Long×short stems

All axial
All long

651 axial; 207 terminal
787 long; 277 short

3.14:1
2.84:1

Review: Concepts of meiosis and the independent assortment of chromosomes
This is the foundation for our studies in genetics and later on, evolution

Recall: Homologous pairs of chromosomes

…And we get our four daughter cells

Our four daughter cells again: The chromosomes carry genes, and the alleles of those genes

Mendel’s pea plants

To create hybrid plants, Mendel snipped off the male stamens from the reproductive organs to prevent self-fertilization. He then used a paintbrush to transfer pollen from another plant for fertilization.

Mendel also did reciprocal crosses: A female of phenotype A is crossed with a male of phenotype B and vice versa

♀A × ♂B ♂A × ♀B

Purple

Parental: PP
Gametes: P only

×

White

pp
p only

Fusion of Gametes

Can we make a Punnett square?

?

F1

P

p

Pp
All purple flowers

In the F1 generation, we have a single phenotype and a single genotype

Now let’s allow the F1 generation to selffertilize to get an F2 generation. The F1 products of meiosis assort independently

F2 Parental: Gametes:
Gives us: P

Pp P p
p

× P
P

Pp p
p

*We’ll use these gamete types to make a Punnett square

F2
P
Eggs

Sperm

P

p

PP Pp

Pp pp

p

3:1 Phenotypic ratio 1:2:1 Genotypic ratio

The selfing, or intercross of identical heterozygous F1 individuals is called a monohybrid cross It was this type of cross that provided the 3:1 progeny ratio that suggested the principle of equal segregation of chromosomes (plus the genes residing on them) into gametes

Mendel then tried to follow the inheritance of two characters at the same time: He wanted to know if two characters, such as seed shape and color, are inherited together as a package or if they assort independently
Will seed color and shape alleles always stay together?

Mendel’s first hypothesis: Dependent Assortment
If the two characters segregate dependently (that is, together), the F1 hybrids can only produce the same two classes of gametes that they received from the parents, and after selfing, the F2 offspring will only show the parental phenotypes in a 3:1 ratio

The two characters would be inherited together (dependent assortment) if the alleles were on the same chromosome together:
This is called linkage Fortunately, Mendel’s experiments were not complicated by linkage

Let’s see how to put a dihybrid cross together into a Punnett square YYRR × yyrr

P Generation: Gametes:

YYRR × yyrr YR × yr

Yy

×

Rr

This gives us all combinations for our Punnett square:

YR, Yr, yR, yr

YR YR Yr yR

Yr

yR

yr

?

yr

YR YR Yr yR

Yr

yR

yr

YYRr
YyRr

yr

YR

Yr

yR

yr

YR YYRR YYRr YyRR YyRr Yr YYRr YYrr

YyRr Yyrr

yR YyRR YyRr yyRR yyRr

yr YyRr Yyrr

yyRr

yyrr

What is a testcross?
We use a testcross to determine the genotype of an organism that displays an ambiguous phenotype; That is, a purpleflowered pea plant can have either the PP genotype or the Pp genotype We can cross the purple-flowered plant with a homozygous recessive plant—a white one with a genotype that is known

When we’re done with the testcross, we will examine the phenotypes of the progeny (F1) If we get all purple-flowered plants, we know unambiguously that the purple parent plant is homozygous dominant—PP

Testcross

Testcrosses can also be done with the fruit fly, Drosophila melanogaster These types of crosses were originally done to test for linkage by looking for recombinant phenotypes

Incomplete dominance

The term “incomplete dominance” can be confusing and misleading: The threephenotype scenario of incomplete dominance is a perfect example of the additive effects of alleles

In this case, “additivity” is a type of dosage effect of a particular allele: Each copy present contributes a discrete amount to the phenotype

Let’s assign the variable CR to represent the allele in snapdragons that codes for a functional pigment, in this case, the color red For every copy of CR present, we get one “dose” of the product needed to create the red pigment When both copies of this allele are present, we get the red phenotype; when one copy is present we get the pink phenotype

When there are no copies of the CR allele present, the phenotype is white

The characters of Mendel’s pea plants are called “discrete” For instance, the flower petals are either purple or white, and the height of the plants are either short or tall These types of “either-or” characters are actually quite uncommon: Most traits are quantitative in that they vary over a continuum

Human height and weight are two examples of continuous traits that vary along a continuum

Many of these kinds of characters are controlled by the effect of many alleles of many genes working together, and each of these alleles add a discrete “amount” to the phenotype of the organism

For instance, there is evidence that skin pigmentation in humans is controlled by at least three separately inherited genes, which we will call A, B, and C Each of these genes has a dark-pigment allele that contributes one “unit” of pigment to the phenotype An AABBCC person would be very dark, while an aabbcc person would be very light

What we get is a continuum of pigmentation in humans that forms a bell-shaped curve

We also have to consider environmental effects (sunlight in particular) that blur the line between the different genotypes

A simplified model for polygenic inheritance of skin color

Homework
Pages 119 – 120 in your lab manual Do questions 1 – 7


				
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