Genetics Terms• Gene = a section of DNA (on a chromosome) that codes

for a specific trait

• Alleles = alternate forms of a gene

• Where are the alleles of a gene located?

• How many alleles can a person inherit for any one trait?

• How many alleles are there in a population for a particular trait such as hair color?

Genetic terms, continued…

• Dominant allele =

• Homozygous dominant =

• Heterozygous =

• Homozygous recessive = aa– Recessive trait is expressed

• Genotype = particular alleles a person carries: AaBBccDd• Phenotype = an individual’s observable traits (hair color,

eye color, height,….

Fig 9.7

Gregor Mendel, the father of genetics

•Each offspring inherits two units (alleles) of information, one from each parent

Fig 9.6 a

True-breeding plants (homo-zygous)

All plants had purple flowers, all heterozygous

Fig 9.6b

Punnett square predicts a phenotypic ratio of 3:1 for the F2 Generation

Monohybrid cross:Pp x Pp

Mendel’s Principle of Segregation

Formation of gametes from a pre-gamete cell

Genotype of Pre-gamete cell:

meiosis

Genotype of Gametes: A a

Aa

The Chromosomal Basis of Mendel’s Principle of Segregation:

Nucleus

Round allele (R)

Wrinkled allele (r) Yellow

allele (Y)Green

allele (y)

Seed color gene

Pea shape gene

Mendel’s Second question: how do 2 pairs of genes (on different chromosomes) assort into gametes? –

Mendel’s 2nd experiment:

Fig 9.08

93

3

1

Fig 9.08

Mendel’s principle of independent assortment

• =

R r

Y y

R

Y

rr

yy

R

R

Ry

y

Y

Y

RY

r

ry rY

(figure not in book)

Laws of Probability—application to inheritance

1. The results of one trial of a chance event do not affect the results of later trials of that same chance event

– E.g. Tossing of a coin, gender of children, etc.

Laws of Probability—application to inheritance

2. The Multiplication Rule:

a. What are the chances of a couple having 4 girls?

b. E.g. What are the chances of a couple having a boy with the following characteristics: • Brown hair (3/4), Non-tongue roller (1/4), Blue eyes

(1/4), Attached earlobes (1/4)

How to Solve Genetics Problems

Sample Problem: Mom and dad are heterozygous for tongue rolling where tongue rolling is dominant to non-rolling. What is the chance that the couple will produce a child that is a non-roller?

Use the following steps as a general guide to solve this and other problems:

1. Select a letter to represent the gene involved:

2. Write the genotypes of the parents.

3. Determine all possible gametes for each parent.

4. Make a Punnett square to represent all possible gamete combinations between the two parents and determine the genotypes of the offspring.

5. Use the genotypes found in the Punnett Square to determine the possible phenotypes of the offspring to answer the question.

Sample problem: A true breeding black mouse was crossed with a true breeding brown mouse to produce the F1 generation, below. The F1 generation was then inbred to produce an F2 generation.

a.) Which allele is dominant? How do you know?

b.) Determine the genotypes and phenotypes for all 3 generations

c.) Predict the genotypic and phenotypic ratios for the F2

Types of genetics problems: Monohybrid Cross

A mouse with black fur was crossed with a mouse with brown fur to produce the F1 generation, below. The F1 generation was then inbred to produce the F2 generation. Dominance is the same as in sample problem #2.

a.) Determine the genotypes and phenotypes for all 3 generations

b.) Predict the genotypic and phenotypic ratios for the F2.

Monohybrid Cross Sample Problem #2

A couple, Jack and Jill, is concerned about having a child with cystic fibrosis. Although both of Jack’s and both of Jill’s parents are healthy and show no signs of cystic fibrosis, both Jack and Jill each had a sister die of the disease. The couple went to a clinic to be genetically tested for cystic fibrosis and were each found to be heterozygous for cystic fibrosis. What are the chances of Jack and Jill having a….

a.) phenotypically healthy child?

b.) child that is homozygous dominant? Heterozygous? Homozygous recessive?

c.) girl with cystic fibrosis? Boy with cystic fibrosis?

Monohybrid Cross Sample Problem #3

Types of Genetics Problems: Dihybrid Cross

Dihybrid Cross Sample Problem #1: True breeding parental pea plants were crossed to produce the F1 generation, below. The F1 generation was inbred to produce an F2 generation.

a.) Which alleles are dominant? How do you know?

b.) Determine the genotypes and phenotypes for all 3 generations

P: Long & purple flowered pea plant x Short & white flowered pea plant

F1: All Long & Purple Flowered pea plants

F2: 9 Long & Purple : 3 Long & White : 3 Short & Purple : 1 Short & White

Genotype ratio for F2:1/16 = YYRR2/16 = YYRr 2/16 = YyRR4/16 = YyRr1/16 = YYrr2/16 = Yyrr1/16 = yyRR2/16 = yyRr1/16 = yyrr

Example

• From the crosses below, what are the chances of producing an organism with all

– dominant phenotypes? – recessive phenotypes? – homozygous dominant genotypes?

1. AaBb x AaBb

2. AaBbCc x AaBbCc

3. AaBBCc x aabbcc

How to use the probability method

1. Treat the problem as if it consisted of several monohybrid crosses

2. Determine the gametes for each of these monohybrid crosses

3. Make a Punnett square for each of the monohybrid crosses

4. Use the information from each Punnett square and the “multiplication rule” to solve the problem

Non-Mendelian Inheritance

• Sometimes there are more than 2 alleles in a population

1. Codominance

• Both alleles expressed together as heterozygotes

Codominance: Blood Types

• Alleles• IA = Allele for Type A• IB = Allele for Type B• i = Allele for Type O

• What do these alleles code for? Different carbohydrate “markers” on the RBCs

Blood Type

(Phenotype)

Surface Molecule on R.B.C.

Possible Genotypes

A IAIA or IAi

B IBIB or IBi

AB IAIB

O ii

Blood Types: Sample Problem #1

A couple has the type A and Type B, respectively. Is it possible for them to have a child with the following blood types?

a. Type O

b. Type A

c. Type B

d. Type AB

Blood Types: Sample Problem #2

A couple has the type A and Type AB, respectively. Is it possible for them to have a child with the following blood types? If so, what is the genotype of each parent?

a. Type O

b. Type A

c. Type B

d. Type AB

Rhesus Factor—a RBC surface molecule

• Rh factor is inherited independently from the ABO system

• Rh positive people:

• Rh Negative people:

• Alleles– R = Rh factor is present– r = no Rh factor present

Phenotype Possible Genotypes

Rh +(Rh positive)

Rh-(Rh negative)

Blood Types: Sample Problem #3A couple has the type A+ (heterozygous) and Type AB+, respectively. What are the chances of the couple having a child with the following phenotypes? Both parents are heterozygous for the rhesus factor.

a. Type O+ b. Type O-

c. Type A+ d. Type A-

e. Type B+ f. Type B-

g. Type AB+ h. Type AB-

2. Incomplete dominance

• One allele isn’t fully dominant over the other

Fig 9.16

3. Continuous variation in a population

• Individuals in a population show a range of small differences in a certain trait

• Ex.

• Causes:

• 1.

• 2. Multiple environmental factors affect gene expression.

Fig 9.211. Polygenic

inheritance:

AABBCCaabbcc

2. Environmental influence on gene expression:Ex.

• Three types of disorders:

1. Autosomal recessive –

2. Autosomal dominant :• AA – embryo dies• Aa – is afflicted• aa – normal individual• Ex. Dwarfism• Vary rare

Human Genetics

3. Sex-linked recessive inheritance

• Recessive on X-chromosome• e.g. • Males usually get the disorder more than females –

why?

• Son cannot inherit a recessive allele from his father but a daughter can. Why?

Human Sex Chromosomes • Sex chromosomes in humans

– Female Genotype = – Male Genotype =

• Sex-linked Alleles are carried on the X-chromosome

• Y-chromosome

– SRY gene on Y chromosome stimulates gonads to differentiate into male sex organs. • SRY = Sex-determining Region, Y-chromosome

Sperm

ParentsMaleFemale

Chromosomessegregate in

meiosis

Offspring

Eggs

Twodaughters

Twosons

Gender Determination in Humans

Sample Problem

Mary’s paternal and maternal grandfathers are both colorblind. There is no evidence of colorblindness in either grandmother’s family histories, and Mary’s father is not colorblind

a. What is Mary’s genotype? Phenotype?

b. What are the chances that Mary’s brother is colorblind?

Other genetic diseases can arise through changes in chromosome structure