SOLVING HYBRID CROSS PROBLEMS Punnett Squares – used to predict the

percentages of genotypes and phenotypes of crosses

Ex. Widows Peak X Straight line Widows Peak is dominant = W

Straight line is recessive = w

Phenotype v. GenotypePhenotype v. Genotype

Phenotype = physical appearance example: widows peak or straight line

Genotype = genetic makeup example: Ww, WW, or ww

Genotypes can be homozygous (purebred)or heterozygous (hybrid)

Homozygous Widows Peak = WW Homozygous Straight line = ww Heterozygous Widows Peak = Ww

What will the genotypes and phenotypes be of a cross of

heterozygous parents?

Ww X Ww (P1 = 1st Parents)

W

w

w F1 = 1st Offspring

WW Ww

Ww ww

Phenotypes

Widows peak =75%

Straight line = 25%

Genotypes

WW =25%ww=25%Ww=50%

W1st Law – ½ chance for each

2nd Law – ½ X ½ = ¼

3rd Law½ X ½ + ½ X ½ = ½

PUNNETT SQUARE PRACTICE

Use a Punnett Square to determine the phenotype and genotype ratios of a cross between:

1. Bb X bb 2. TT X tt3. Tt X Tt

RESULTS OF PUNNETT SQUARES

TtTt TtTt

TtTt TtTt

BbBb BbBb

bbbb bbbb

b b

T T

B

t

b

t

1.

2.

PUNNETT SQUARE RESULTS

TTTT TtTt

TtTt tttt

T

3.tT

t

DIHYBRID CROSSES – TESTS HOW TWO DIFFERENT TRAITS ARE INHERITED

Led to Principle of Independent Assortment - genes for different traits

usually segregate into gametes independently of one another.

TWO-FACTOR CROSS

Cross two plants that are heterozygous for both pod color (G) and height (T)

TtGg X TtGg

Meiosis produces the following gametes:TG, Tg, tG, tg

T = tall G = greent = short g = yellow

TTGG TtGG TTGg TtGg

TtGG

ttGG TtGg ttGg

TTGg TtGg TTgg Ttgg

TtGg ttGg Ttgg ttgg

TG tG Tg tg

TG

tG

Tg

tg

Red = tallBlack= short

BEYOND DOMINANT AND RECESSIVE ALLELES Incomplete Dominance – Cases in

which one allele is not completely dominant over another. The heterozygous phenotype is somewhere in between the two homozygous phenotypes.

Ex. Four o’clock plants: Homozygous are white or red Heterozygous are pink

BEYOND DOMINANT AND RECESSIVE ALLELES Codominance – Both alleles contribute to

the phenotype of the organism.Ex. Hair color in cattle:

Homozygous are red or white Heterozygous are roan (pinkish

brown) – a mixture of red and white hairs

CROSS 2 ROAN CATTLE

R W

R

W

RR (red)Rr (Roan)

Rr (Roan) Rr (White)

BEYOND DOMINANT AND RECESSIVE ALLELES Multiple Alleles – Many genes have more

than two alleles. An individual organism has only two alleles, but more than two possible alleles exist in the population.Ex. 1. Coat color in rabbits:

Four alleles exist for one gene. This results in four possible coat

colors.Ex. 2. Genes for blood type in humans

ABO BLOOD GROUP

Blood type is an example of a trait with multiple alleles: A, B, O

A and B are co-dominant, O is recessive Type O is the Universal Donor – all blood

types can receive type O blood. Type AB is the Universal Recipient – can

receive blood from all other blood types.

GENOTYPES & PHENOTYPES

Genotype Phenotype

AA or AO Type A

BB or BO Type B

AB Type AB

OO Type O

PRACTICE PUNNETT SQUARES FOR BLOOD TYPE INHERITANCE

1. What is the probable genotype ratio among children born to an AO mother and an AB father?

2. One parent has type A blood and the other has type B. What are their genotypes if they produced children who were:

1. All AB2. ½ AB and ½ A3. ¼ AB, ¼ A, ¼ B and ¼ O

BEYOND DOMINANT AND RECESSIVE ALLELES Polygenic Traits – Traits controlled by two or

more genes.Ex. 1. Three genes control eye color in fruit

flies. Different combinations of alleles for these genes produce different eye colors.

Ex. 2. Skin color in humans is probably controlled by more than four genes.

Ex. 3. Eye color in humans

GENES AND THE ENVIRONMENT

Environmental conditions can affect gene expression and influence genetically determined traits Ex. Western White Butterfly – those that hatch

earlier have more pigment in wings – increases body temperature so they can fly

GENE LINKAGE

Alleles of different genes on the same chromosome tend to be inherited together – linkage groupsThis was missed by Mendel – he happened to

study traits located on different chromosomes.Led to clarification that chromosomes assort

independently Alleles of genes far apart on chromosomes can

assort independently because of crossovers. Frequency of crossovers can be used to determine

location of genes on chromosome

The more frequent recombination is between two genes, the more likely those genes are far apart on the chromosome.