Patterns of Inheritance
I. Meiosis and Gametogenesis (Ch. 13)
Genetics is the study of heredity and variation. Heredity is the continuity of biological traits
from one generation to the next. These traits are passed from parents to offspring in genes on
chromosomes. Variations are the inherited differences among individuals of the same species
Like begets like…more or less.
B. Comparing Sexual and Asexual Reproduction
Each parent passes on half its genes to offspring
Offspring are a genetic clone. Diversity can occur
only as a result of mutation
Alternation of meiosis and fertilization is common to all sexually reproducing organisms.
C. Why isn’t mitosis enough?
The body or ____________________________ cells in a sexually reproducing organism are
said to be _________________________, meaning they get one set of chromosomes from the
male parent and one set of chromosomes from the female parent. In humans, we get 23
chromosomes from mom, and 23 corresponding or matching chromosomes from dad. These
corresponding chromosomes are ___________________________________, or paired
because they carry genes for the same information.
Homologous chromosomes are a pair of chromosomes of the same length, centromere
position, and staining pattern that possess genes for the same characteristics at corresponding
In sexual reproduction, the gametes from opposite sexes unite to form a _____________.
Because the zygote is diploid (in the case of humans, it has 46 or 23 pair of chromosomes), the
number of chromosomes in gametes must be ___________________. This occurs during the
process of meiosis.
Meiosis consists of two successive nuclear divisions. It is a process of reduction division
in which the number of chromosomes per cell is cut in half and the homologous chromosomes
that exist in a diploid cell are separated. This creates ____________________________ cells.
D. Stages of Meiosis
Meiosis I (the * shows how the phase differs from the phase during mitosis)
same as mitosis except
* homologous chromosomes pair up to form tetrads, & crossing-over occurs. Sites of
crossing over are called
pairs of homologous chromosomes line up at the center of the cell.
* homologous chromosomes, each made of 2 chromatids, stay together.
homologous chromosomes separate move to opposite sides of the cell.
* Chromatids do not separate at their centromeres. Nondisjunction, or failure
of chromosomes to separate evenly into 2 cells, can occur here.
chromosomes are gathered at the two opposite poles. They are still duplicated and
chromatids are still connected at centromeres. Cytokinesis occurs and there are now 2
daughter cells, each with 1/2 the number of chromosomes as the original cell.
Meiosis II is essentially just like mitosis. The result is 4 _______________________ (1/2 the
number of chromosomes) daughter cells, that are NOT genetically identical to the original cell.
E. Meiosis & Genetic Variation
The process of meiosis and sexual reproduction provides genetic variation for the species
in 3 ways:
1. _________________ ____________________ - exchange of corresponding segments
of homologous chromosomes in prophase I
2._________________ ____________________ ______ ________________________
- how homologous chromosomes are arranged in metaphase I
3. ______________________ ________________________________- the variety of
gametes produced in two individuals can combine in almost limitless ways.
Inheritable variation is the basis for Charles Darwin’s theory that natural selection is the
mechanisms for evolutionary change. A genetic variation that makes an organism better
suited to its environment will allow the organism to live longer, produce offspring,
thereby passing on its genetic traits.
An organism has 6 chromosomes in its somatic cells. Draw the chromosomes and spindle fibers in the cells
below. For meiosis, show 1 point of crossing over, making sure to carry it through.
prophase metaphase anaphase telophase
prophase I metaphase I anaphase I telophase I/
prophase II metaphase II anaphase II telophase II/
F. Comparing Mitosis and Meiosis
1 produces haploid daughter cells unlike parent cell
involves one cell division 2
produces two daughter cells 3
4 homologous chromosomes pair then separate
individual chromosomes line up at metaphase plate 5
no crossing over occurs 6
7 needed for sexual reproduction
II. Inheritance Patterns (Ch. 14)
A. Gregor Mendel
The formal study of genetics began with the work of a 19th-century monk named ____________
Mendel began breeding and observing pea plants in his Augustinian monastery in Brno
(currently Czech Republic) in the mid 1800’s. He studied 7 traits in peas to see how they were
passed from one generation to the next.
Steps to Mendel’s Experiments
1. Allow peas to self-pollinate for a few generations. You get a pure organism. This is
called _______________, or ____ generation.
2. Cross-pollinate 2 varieties with contrasting traits. This is called __________________
_________________, or ______ generation.
3. Allow F1 generation to self-pollinate. Their offspring are the __________ generation
Traits did not blend. One trait was _______________________(expressed in F1) and one trait was
______________________ (not expressed in F1). A capital letter indicates the dominant trait, and
the same lower case letter indicates the recessive trait.
F1 - All 1 trait (dominant one)
F2 - 3:1, dominant: recessive
What did Mendel’s results mean?
B. Mendel’s Principles or Laws
1. Parents pass on “factors” to their offspring that will determine their traits (these factors are
_______________ - a segment of DNA that codes for a particular protein).
2. A sexually reproducing organism has 2 factors (genes) for each trait - one from mom & one
from dad. These genes may have the same information (homozygous) or they may have different
information (heterozygous). A form of a gene for a trait is called an allele.
3. Principle (Law) of _____________________________ - the two forms of each gene (the
alleles) separate when gametes are formed (meiosis).
4. Principle (Law) of __________________________ ______________________________ - the
genes for different traits separate independently of one another during gamete formation. (if they
are on different chromosomes).
Geneticists now call the genetic makeup of an organism its ____________________. The outward
appearance is the ________________. The phenotype may not always reflect the genetic make-up
C. Probability and Genetics Problems
Mendel’s laws of segregation and independent assortment reflect the same laws of probability that apply to
tossing coins or rolling dice. The probability scale ranges from zero (an event with no chance of occurring)
to one (an event that is certain to occur).
The probability of tossing heads with a normal coin is 1/2.
The probability of rolling a 3 with a six-sided die is 1/6, and the probability of rolling any other number is 1
- 1/6 = 5/6.
When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss.
Each toss is an independent event, just like the distribution of alleles into gametes.
Like a coin toss, each ovum from a heterozygous parent has a 1/2 chance of carrying the dominant allele
and a 1/2 chance of carrying the recessive allele. The same odds apply to the sperm.
1. Rule of multiplication (and)
What is the likelihood of two more events occurring together? (this AND this)
Compute the probability of each independent event.
Then, multiply the individual probabilities to obtain the overall probability of these
events occurring together.
2. Rule of addition (or)
Under the rule of addition, the probability of an event that can occur two or more
different ways is the sum of the separate probabilities of those ways. (this OR this)
What is the chance that two coins tossed at the same will land heads up?
What is the probability that you could roll a die three times in a row and get all threes?
For fun on Saturday night, you and a friend are going to flip a fair coin 10 times
(geek!). You flip HTHHTHTTTH. Your friend flips HHHHHHHTTT.
Which sequence is more likely to occur?
If you roll a die, what is the probability that you will get a 2 or a 6?
D. The Punnett Square
Steps for Using a Punnett Square
1. Draw the square.
2. Decide what alleles can be in the gametes of each parent.
3. Write the letters that stand for the possible alleles in one gamete along the side of the square.
4. Write the letters that stand for the possible alleles in the other gamete along the top of the
5. Copy the letters into the boxes below each letter on the top.
6. Copy the letters into the boxes beside each letter along the side.
7. Look at the small boxes in the large square. These show the possible
combinations for offspring.
To predict the outcome for offspring:
probability of a # of 1 kind of outcome
particular outcome = total # of outcomes
E. The Monohybrid Cross
a cross between 2 organisms to study the inheritance of a single trait.
F. The Dihybrid Cross
a cross between 2 organisms to study the inheritance of 2 traits.
G. Relationship Between Genotype and Phenotype - Variations in Mendelian Genetics
We have seen that some traits are dominant - they will be expressed even if the trait comes from
only 1 parent. Some traits are recessive - you must get the trait from both parents for it to be expressed.
Dominance does not mean an allele subdues another one at the level of the DNA - it is a result of how a
trait is expressed phenotypically. Dominant alleles are not necessarily more common, and recessive alleles
There are also variations to Mendelian gene expression.
1. ____________________ ___________________ - occurs when alleles of a gene are neither
dominant nor recessive - the phenotype of offspring will be intermediate.
Example #1 - combining red & white snapdragons give offspring that are pink. Alleles are CR &
Since heterozygotes can be distinguished from homozygotes by their phenotypes in
incomplete dominance, the genotypic and phenotypic ratios are the same; 1:2:1.
Example #2 - Sickle cell anemia. Alleles are HbA and HbS. HbAHbA = normal hemoglobin.
HbSHbS = all sickle cell hemoglobin. HbAHbS = a blend of normal & sickle cell hemoglobin, or
sickle cell trait. This is a selective advantage in Africa, where it protects against malaria.
2. ___________________________ - both alleles for a gene are expressed (show up) when
present. There is no blending of the traits.
3. ___________________________ - a gene that has three or more alleles.
An example of both codominance and multiple alleles is - ABO blood types.
ABO Blood Types
There are 4 blood types: A, B, AB, O. These letters refer to the A and B carbohydrates
found on the red blood cells - O means no carbohydrate is on the RBCs. There are 3 genes that
A B A
control blood types: I , I , and i. You will get 2 genes, one from your mom, one from your dad. I
and I are codominant; i is recessive to both.
Phenotype (blood type) Genotype
4. ____________________________ - traits that are controlled by two or more genes. You tend to
get a continuous variation of phenotypes. Examples are height, weight, skin color.
5. ____________________________ - the ability if a single gene to have multiple phenotypic
effects. Ex. sickle cell allele deforms the red blood cells, starting a cascade of symptoms
throughout the body.
6. ____________________________ - Interaction between two non-allelic genes in which one
modifies the phenotypic expression of another.
Nature vs. Nurture - Environmental conditions can influence the phenotypic expression of a gene,
so that a single genotype may produce a range of phenotypes. Example - nutrition can influence
height, experience can affect performance on intelligence tests.
III. The Chromosomal Basis for Inheritance (Ch. 15)
A. The Chromosomal Theory of Inheritance
Formulated around 1902. States that Mendelian genes have specific loci (locations) on
chromosomes, and it is the chromosomes that undergo segregation and independent assortment..
The main scientist who provided convincing evidence that Mendel’s inheritable factors are located
on chromosomes was Thomas Hunt Morgan, who worked with fruit flies.
________________________ a chromosome that is not directly involved in determining sex; not a
______________ _____________________ a chromosome responsible for determining the sex of
B. Discovery of Sex-linked Genes
1. Sex-linked genes
Are genes that are located on a sex chromosome – the X chromosome. Fathers pass sex
linked alleles to all of their daughters, none of their sons. Mothers can pass sex-linked alleles to
daughters or sons.
2. Sex-linked disorders
Are almost exclusively due to recessive alleles. For females to have the disorder, they
would have to have 2 defective X’s – be homozygous. With one affected X, they are carriers.
Males, however, have only 1 X, so if it carries a defective allele for a sex-linked trait, he will have
the disorder. Males cannot be carriers.
Color blindness – XB – normal allele; Xb = allele with defect for colorblindness
Hemophilia - XH – normal allele; Xh = allele with defect for hemophilia
C. Tracking Family Traits – Analyzing Pedigrees
In order to learn about an inherited trait, scientists look at family histories, called pedigrees. By
doing this, it can be determined if the trait is dominant or recessive, and if it is sex-linked or autosomal.
How to Analyze a Pedigree
1. Is the trait sex-linked or autosomal?
- If it is sex-linked, it is usually seen only in males.
2. Is the trait dominant or recessive?
- If it is dominant, every person with the trait will have a parent with the trait. If it is
recessive, a person with the trait can have normal parents (with heterozygous genotype).
3. Is the trait determined by a single gene, or several?
- If it is determined by 1 gene, children will be affected by ~ 3:1 ratio when parents are
D. Linked Genes
Genes located on the same chromosome tend to be linked in inheritance and do not assort
independently. They are called ___________________ _______________. Since independent assortment
does not occur, you will not get the predicted ratios in offspring for a dihybrid cross.
E. Genetic Recombination
Genetic recombination is the production of offspring with new combinations of traits different
from those combinations found in the parents. This occurs because of the events of meiosis (independent
assortment & crossing over) and random fertilization
1. recombination of ______________________ genes - independent assortment of chromosomes
2. recombination of ______________________ genes - crossing over
This is due to crossing over in meiosis. If genes are totally linked on chromosomes, there should
be no genetic recombination; in a test cross of heterozygous x homozygous recessive, you would
get a phenotypic ratio of 1:1 of parental phenotypes only.
If genes are unlinked, the offspring of the above test cross shows a 1:1:1:1 ratio of all phenotypes,
with half the same as the parental, & half different. The offspring with different phenotypes are
called recombinants, and the recombination frequency is 50%.
You can calculate the recombination frequency with the following formula:
recombination # recombinants
frequency = # total offspring x 100
A linkage group is two or more genes located close enough together on the same chromosome that
they tend to be inherited together.
3. mapping genes
Recombination data can be used to map a chromosome’s genetic loci. The probability of
crossing over between 2 genes is directly proportional to the distance between them.
The distance between genes is measured in map units, and on map unit (or centimorgan, in honor
of Morgan), is equal to 1% recombination frequency.
Determine the sequence of genes along a chromosome based upon the following recombination
frequencies: A-B = 8; A-C = 28; A-D = 25; B-C = 20; B-D = 33
E. Chromosomal Disorders
Although mistakes in the process of duplicating genetic information & transmitting it to the next
generation are rare, they do happen. These mistakes are called mutations, and they can occur on the
chromosome or in the gene.
1. Alteration of chromosome __________________
_________________________________ - failure of homologous chromosomes to separate during
meiosis I. Two of the resulting gametes will have an extra chromosome (trisomy), and two will be
short a chromosome (monosomy).
One of the most common trisomies is trisomy 21, or Down syndrome.
Klinefelter syndrome - nondisjunction in the sex chromosomes - XXY
Turner syndrome - also nondisjunction in the sex chromosomes - XO
2. Alteration of chromosome ___________________
a. __________________- loss of part of a chromosome.
b. __________________- a segment of chromosome is repeated.
c. __________________- part of a chromosome becomes oriented in the reverse of its
d. __________________- part of one chromosome breaks off and attaches to another,
A ____________________________ is a picture of the chromosomes in a cell at metaphase. They are
arranged in order by size, centromere position, and banding pattern. chromosomal abnormalities can be
detected by looking at a karyotype.
Chromosomal abnormalities in a baby can be determined before birth. There
are two methods commonly used:
a. _______________________________ - amniotic fluid containing embryo cells is
removed from the sac surrounding the developing embryo using a needle. A karyotype of
the chromosomes in the cells is made.
b. __________________ __________________ ___________________ - a sample of
embryonic cells is removed from the membrane surrounding the embryo. A karyotype is
made using these cells.