Genetics

Core Concepts • Mendel’s Law of Segregation states that there are two alleles

for every gene determining a specific characteristic, and these

alleles are segregated into separate gametes during

reproduction.

• When the 2 different alleles occur together in one individual

(heterozygote), the dominant allele will be the one that is

expressed in the phenotype of the individual.

• Mendel’s Law of Independent Assortment says that 2 or

more different genes, if found on separate chromosomes, are

determined independently of each other.

• The Addition Rule is used to combine probabilities, when

there are 2 or more ways to arrive at a given outcome.

• The Multiplication Rule is used to combine probabilities of 2

or more different events that need to occur in combination.

• Many characteristics have been found to follow patterns of

inheritance that are modifications of Mendel’s rules.

Keywords

• gene

• allele

• dominant

• recessive

• character

• trait

• phenotype

• genotype

• pure breeding

• hybrid

• homozygous

• heterozygous

• P (parental generation)

• F1 (first filial generation)

• F2 (second filial generation)

• Punnett

Square

• forked line

Mendel’s

Discoveries • Pre-Mendel beliefs in

genetics: blending theory

• Augustinian monk and science teacher

• Why Pisum sativum? – Pea plants available in many

varieties • character (heritable feature)

• trait (character variant)

– Perfect flowers • cross-pollination and self-

pollination

– Short generation time

– Many offspring

CHARACTERS TRAITS

P Generation (true-breeding parents)

Purple flowers

White flowers

x

F1 Generation (hybrids)

All plants had purple flowers

F2 Generation

705 plants 224 plants

self-pollination

MONOHYBRID CROSS inheritance of a single trait

Cross-

fertilization

Results for other hybrid (F1) characters

Blending

inheritance

Complete

dominance

Mendelian

Inheritance Complete dominance

• Alternative

versions of genes

exist (alleles)

– Dominant

– Recessive

• Organism inherits

2 alleles

– Dominant is fully

expressed

– Recessive has no

visible effect

Allele for purple flowers ( P )

Locus for flower-color gene Homologous pair of chromosomes

Allele for white flowers ( p )

• Alternative

versions of

genes exist

(alleles) • Dominant

• Recessive

• Organism

inherits 2

alleles

(genotype) • Dominant is

fully expressed

• Recessive has

no visible effect

PP pp

Pp

PP or

Pp pp

Other terms in

genetics

• Homozygous – identical

alleles, true-breeding

• Heterozygous – different

alleles

• Phenotype – physical

appearance

• Genotype – genetic make-up

Mendelian

Inheritance Law of Segregation

2 alleles for one

character

separate and go

to different

gametes

Parental generation

• Pure-breeding parents are homozygous for

different traits for flower color

• Each parent produces only one type of

gamete: purple parent (P), white

parent (p)

F1 generation

• Hybrids are heterozygous for flower color

• Can produce two kinds of gametes: P

or p

F2 generation

• Due to the law of segregation, eggs and

sperm with different kinds of gametes

combine to form a specific ratio

• Genotypic ratio 1 PP : 2 Pp : 1 pp

• Phenotypic ratio 3 purple : 1 white

Problem-solving An organism displays the

dominant phenotype. It

may be either homozygous

or heterozygous for the

trait.

How can its genotype be

determined?

Perform a test-cross:

Dominant phenotype x

recessive phenotype

Dihybrid cross Inheritance of two characters

• Produces

four

phenotypes

in the F2

generation

Dihybrid cross Inheritance of two characters

• Characters are

NOT

transmitted as a

package (YR

and yr only)

• Genes are

packaged into

gametes in all

possible

combinations of

alleles (YR, Yr,

yR, yr)

Law of independent

assortment Allele pairs separate independently

during the formation of gametes

Genetics and Probability

Addition Rule

combines probabilities for mutually exclusive

events

Multiplication Rule

combines probabilities of 2 or more independent

events that need to occur together

Monohybrid cross Round seed shape is dominant over wrinkled seeds in pea plants.

What percentage of the F2 generation can be expected to:

a. Be homozygous dominant for seed shape?

b. Be heterozygous for seed shape?

c. Have round seeds?

Genetics and Probability

Addition Rule

combines probabilities for mutually exclusive

events

Multiplication Rule

combines probabilities of 2 or more independent

events that need to occur together

Dihybrid cross Round seed shape is dominant over wrinkled seeds in pea plants. Green pod color is dominant over yellow pods.

What percentage of the F2 generation can be expected to:

a. Be heterozygous for both characters?

b. Have round seeds in green pods?

c. Have wrinkled seeds in green pods?

Genetics and

Probability

Addition Rule

combines probabilities for mutually exclusive

events

Multiplication Rule

combines probabilities of 2 or more independent

events that need to occur together

Trihybrid cross Round seed shape is dominant over wrinkled seeds in pea plants. Green pod color is dominant over yellow pods. Tall plants are dominant over short ones.

What percentage of the F2 generation can be expected to:

a. Be heterozygous for all characters?

b. Be tall plants with round seeds in yellow pods?

c. Be short plants with round seeds in green pods?

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Monohybrid cross problems 1. In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a genetic cross of

two plants that are heterozygous for the seed shape trait, what fraction of the offspring

should have spherical seeds?

2. In pea plants, yellow seed color is dominant to green seed color.

A. If a heterozygous pea plant is crossed with a plant that is homozygous recessive for seed

color, what is the probability that the offspring will have green seeds?

B. If all of the offspring of a particular cross have the genotype Yy, what must the genotype

of the parents be?

C. To identify the genotype of yellow-seeded pea plants as either homozygous dominant

(YY) or heterozygous (Yy), you could do a test cross with plants of genotype _______.

3. In rabbits, black fur (B) is dominant to white fur (b). Perform the following crosses. For

each cross, give the phenotype and genotype of all offspring.

A. BB x bb B. Bb x Bb C. BB x Bb

4. In summer squash, white fruit color (W) is dominant over yellow fruit color (w). If a

squash plant homozygous for white is crossed with a plant homozygous for yellow, what

will the phenotypic and genotypic ratios be for:

A. the F1 generation? B. the F2 generation?

C. What will the phenotypic and genotypic ratios of the offspring be if you perform a

testcross with the F1 generation?

Dihybrid and trihybrid cross problems 1. The ability to roll one’s tongue is dominant over non-rolling. The ability to taste

certain substances is also genetically controlled. For example, there is a substance

called phenylthiocarbamate (PTC for short), which some people can taste (the

dominant trait), while others cannot (the recessive trait).

A woman who is both a homozygous tongue-roller and a non-PTC-taster

marries a man who is a heterozygous tongue-roller and is a PTC taster, and they

have three children: a homozygous tongue-roller who is also a PTC taster, a

heterozygous tongue-roller who is also a taster, and a heterozygous tongue-roller

who is a non-taster.

A. If these parents have more children so that they had 12 in all, how many of

those 12 would you expect to be non-tasters who are homozygous for tongue-

rolling?

B. If the first child (the homozygous tongue-roller who is also a PTC taster)

marries someone who is heterozygous for both traits, draw the Punnett square

that predicts what their children will be.

2. Using the forked-line method, determine the genotypic and phenotypic ratios of

these trihybrid crosses:

A. AaBbCc x AaBBCC B. AaBBCc x aaBBCc

Non-mendelian inheritance

1. Codominance – two dominant alleles affect

phenotype in separate ways

– both alleles manifest

– e.g. roan coloring in horses – both red and white

2. Incomplete dominance – phenotype of offspring is

between phenotypes of two parents

– e.g. red and white parents give rise to pink offspring

3. Multiple alleles – genes with more than two

alleles that control the phenotype

– e.g. ABO blood type system has 3 alleles--A,B,i. A and B are codominant, i is recessive to both

Sample problems

1. A cross between a blue blahblah bird & a white blahblah bird produces silver blahblah birds.

a) What are the genotypes of the parent blahblah birds in the original cross?

b) What is the genotype of the silver offspring?

c) What would be the phenotypic ratios of offspring produced by two silver blahblah birds?

2. The color of fruit for plant "X" is determined by two alleles. When two plants with orange fruits are crossed the following phenotypic ratios are present in the offspring: 25% red fruit, 50% orange fruit, 25% yellow fruit. What are the genotypes of the parent orange-fruited plants?

3. Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a red roan bull.

4. What should the genotypes & phenotypes for parent cattle be if a farmer wanted only cattle with red fur?

Sample problems

5. A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together).

a) What pattern of inheritance does this illustrate?

b) What percent of kittens would have tan fur if a tabby cat is crossed with a black cat?

6. A woman with Type O blood and a man who is Type AB are expecting a child. What are the possible blood types of the kid?

7. What are the possible blood types of a child whose parents are both heterozygous for "B" blood type?

8. What are the chances of a woman with Type AB and a man with Type A having a child with Type O?

9. Determine the possible genotypes & phenotypes with respect to blood type of the offspring of a couple whose blood types are A and B. Both sets of grandparents have the AB blood type.