Chapter 14

Genetics

Introduction

• Pre 1800s – blending hypothesis

• 1850s – Gregor Mendel

Pisum sativum – what makes pea good genetic model?

• Pea character (gene) =

• Pea trait (allele) =

• self fertilization

Vs. cross fertilization

• What is emasculation?

• What are the embryos of the pea?

The monohybrid cross

• Cross true breeding parental plants (P)

• produces F1 generation

-character (gene) -trait (allele) -ratio of phenotypes in F2 - dominant trait

F1 plants all purple

• Cross F1 plants

F2 plants = ~250 white ~750 purple

A monohybrid

Is this quantitative or qualitative data?

Mendel’s Conclusions

Dominance

Allele = Alternative version of gene

Dominant allele P Recessive allele p

Each individual inherits 2 alleles/gene, 1 from each parent

PP homozygous dominant phenotype?

pp homozygous recessive

Pp heterozygous

Dominant allele masks expression of recessive allele

Find the dominant allele Pick symbols

What is the phenotype of plant with genotype Rr? What is genotype of a green plant?

Genotype?

Segregation

• The 2 alleles segregate in gametes during meiosis

• Random segregation

Human chromosome 11 gene for beta globin Genotype Hbb/Hbb

Pea chromosome 1 Genotype of plant? Phenotype of plant?

• Mendel did not know about:

– DNA/chromosomes/genes

– Crossing over or the cell cycle

– Linked alleles

try it

In mice, black fur is dominant over white fur

Cross heterozygous mouse with homozygous recessive mouse

In mice, black fur is dominant over white fur Cross heterozygous mouse with homozygous recessive mouse

1. make an allele key

2. Show the cross

3. Write the phenotypes under the genotypes

4. Determine the parental gametes (random segregation)

5. Add to Punnett square

6. Determine offspring

Punnett square

B

b

b b

Bb Bb

bb bb

What is the expected ratio of phenotypes? Genotypes? In a litter of 3, how many are expected to be white?

Test cross – mate to homozygous recessive individual

• In cats striping is dominant over non-stripes. Mate a homozygous striped cat with a non-striped cat. What % are expected to be striped? S= stripes s = not

The Scottish fold is a dominant trait

Genotypes FF Ff ff

Phenotypes?

The original Scottish Fold was a white barn cat named Susie, found in Scotland

Test cross

We have a Scottish fold cat

Is she Ff or FF?

If FF, then cross to ff kittens?

If Ff, then cross to ff kittens?

Review • All possible one gene crosses B = black fur allele b = white fur allele • BB X BB all black • BB X Bb all black • BB X bb all black • Bb X Bb • Bb X bb • bb X bb all white

The Dihybrid cross (2 genes)

Independent Assortment 2 genes

Y = yellow pea allele R = round pea allele

y = green pea allele r = wrinkled

Phenotypes of peas

True-breeding yellow, round X true-breeding green, wrinkled

Allele key?

P Genotype?

Gametes? (always 4 per parent when 2 gene cross)

X

R = round allele Y = yellow r = wrinkled y = green

rryy RRYY

RY and ry

• RRYY X rryy

• Gametes? • Offspring: RrYy all round, yellow

• RrYy X RrYy gametes? RY, Ry, rY, ry

Round round Yellow yellow

F1 generation

RY

ry

RrYy

These dihybrid plants are heterozygous for both traits

• Cross two dihybrid F1 plants

RrYy X RrYy Gametes: Percentage of each gamete type?

F2 generation

Ratio of genotypes? Ratio of phenotypes?

• The two genes assort independently during meiosis (Mendel’s genes were on different chromosomes)

Concept check pg 268: Pea plants heterozygous for

flower position and stem length are allowed to self pollinate. How many of 400 offspring would be predicted to have terminal flowers and be dwarf? Use Punnett square.

A = normal T = axial flowers a = dwarf t = terminal flowers

Extending Mendelian Genetics

1. incomplete dominance

• Neither allele is dominant

• Ex. CR allele = red flower CW allele = white flower

Phenotype of CR CW plant =

Cross 2 F1 plants F2?

2. Codominance

Both alleles expressed in phenotype

Ex. Blood type I gene

Alleles IA IB

Phenotype of IA IB ?

• 3. multiple alleles

Ex. blood type

Alleles IA IB i

IA IA IB IB IA IB

IAi IBi ii

4. polygenic inheritance

quantitative characters

multiple genes affect characteristic

Ex. skin – at least 3 genes

Height

• 5. environmental effects

A genotype has a norm of reaction (phenotypic range)

• Ex

– Blood type has no norm

– Skin color has a norm

Chapter 14 genetics problems

1, 2, 3, 5, 6, 8, 14, 16, 17.

These types of problems will be on exam 2

Human traits

• Pedigree analysis

?

Recessively inherited disorders A allele normal protein a allele malfunctioning or no protein AA Aa heterozygotes are carriers aa

disorders range from fetal lethal to mild

Ex. albinism a allele does not code for functional melanin protein

Chromosome 11

Recessive disorders

ss = sickle cell disease (chromosome 11)

cc = cystic fibrosis (chromosome 7)

tt = Tay Sachs (chromosome 15)

Recessive traits

bb = blond hair

ii = type O blood

Dominantly inherited disorders A allele abnormal protein

a allele normal protein

AA

Aa

aa

Achondroplasia D allele causes dwarfism

How could two normal height parents have a child with achondroplasia?

Dominant disorders

Hh = Huntington disease (chromosome 4)

Pp or PP = polydactyly (chromosome 7)

Multifactorial disorders

Genetic component + environment (aging, diet, chemicals, radiation, etc.)