Genetics

Review

• Chromosomes are paired (homologous) in a 2N organism.

• Each chromosome contains many genes

• Homologous chromosomes carry the same genes.

Homologous pairof chromosomes

Review

• Genes can have different versions of the same trait. These versions are called alleles.

• Homologous chromosomes separate into different gametes.

alleles

Alleles - Dominant and Recessive

• Each parent will donate one allele.

• Alleles can be physically apparent (dominant) or physically hidden (recessive).

• Mendel worked with traits that had only two alleles of each trait, one dominant and one recessive.

Mother Father

Alleles

Mendel’s Laws

• The Law of Segregation– each pair of alleles separates during meiosis

(gamete formation)

• The Law of Dominance– When two alleles differ, – one can control the trait - Dominant – the other can be hidden - Recessive

• The Law of Independent Assortment– Pairs of genes segregate into gametes randomly

and independently (due to random separation of homologous pairs.

How Can We Demonstrate these Laws?

• Mendel recorded phenotypes ( form of the trait that he “saw”)

• Based on outcomes from matings (called crosses) over several generations, he could determine the genotypes (the actual genetic makeup for that trait) of previous generations– Homozygous - both alleles for a trait are the same

– Heterozygous - alleles are not identical.

Monohybrid Crosses

• Studying the inheritance of a single trait while ignoring all other traits

• Start with 2 “pure parents”

– P Generation• GG (only green)• gg (only yellow)

– F1 generation

• All green heterozygous

Determining Genotype (1 Trait)

green yellow

4 All green

Self- fertilized

Self fertilization of this generation produces a different pattern for homozygous vs. heterozygous

Cannot see the yellow in this generation

Yellowreturns

PARENT

F1

F2

Using a Punnett Square instead

• Grid for Organizing Genetic Information

• Shows Probability that a combination will occur

1) Determine Parent alleles

2) Place alleles on edge of square

3) Combine alleles inside the boxes

4) Determine genotypes and phenotypes

G

G

g g

Parent 1

Parent 2

Gg

Gg Gg

Gg

Possible offspring genotypes for F1

Heterozygous cross

• Phenotypes vs. Genotypes– The recessive trait will show ONLY in the absence of

the dominant (G) trait.G g

G

g

GG Gg

gggG

GG : Gg or gG : gg 1 : 2 : 1

Green Yellow3 : 1

GENOTYPE

PHENOTYPE

Test Crosses

• Purpose - – To distinguish between a homozygous dominant

and a heterozygous dominant

– Cross the unknown with a known homozygous recessive.

– Create two Punnett squares.• Cross a homozygous dominant with a homozygous

recessive• Cross a heterozygous dominant with a homozygous

recessive• How are the patterns different?

Heterozygous + Homozygous Recessive• Phenotypes vs. Genotypes

– The recessive trait will show ONLY in the absence of the dominant (G) trait. What will happen?

g g

G

g

GG :Gg or gG : g: :

Green Yellow :

GENOTYPE

PHENOTYPE

GENOTYPE

Predicting any single outcome

• Given Mendel’s Laws, the probability of an outcome for any one combination can be estimated.

• Can Use coins to estimate probability– See Exercise on Probability.

Probabilities and Proportions

• Should approximately match.

• Are only estimates of what SHOULD happen.

• Can be easily changed If “randomness” is compromised.

Dihybrid crosses

• Allow the study of two traits at a time.

RY RY

ry

ry

Make a P cross (RRYY x rryy)

Cross F1 plants among themselves (RrYy x RrYy)

List all gametes on a 4 x 4 Punnet Square

Intermediate Inheritance

• Mendel’s study – Dominant and recessive

• Many genes have intermediate inheritance– Heterozygous phenotype not like either

homozygous phenotype

Types of Intermediate Inheritance

• Incomplete Dominance

• Heterozygote phenotype is “inbetween “ the two homozygous phenotypes

– EXAMPLE: Red + White = Pink Flowers

• BUT: Gametes will contain either Red or White!• No dominant phenotype so only capital letters with

superscripts are used.

• Red = FrFr White = FwFw Pink = FrFw

Types of Intermediate Inheritance

• Codominance

• Both alleles express themselves fully– EXAMPLE: Blood types

• A, B, AB

Polygenic Traits

• Trait is controlled by two or more gene pairs

• Eye color, skin, hair– Tone amount, and distribution of pigments are all

factors. Each is controlled by a different gene pair, for each attribute (color, skin, hair)

Multiple alleles

• Three or more alleles in a trait• Blood type

– IA IB and i– Both IA and IB are dominant over I– IA and IA are codominant.– NOT THE SAME as polygenic

• A single pair of genes with > 2 alleles.

Environmental Stress on Genetics

• Seasonal Changes( external changes)• Turning on “beard growth (internal

changes)

• Genes can be turned on and off.