GENETIC CROSSES

REVIEWDo Now: Do the dihybrid cross on

your notes sheet.

DIHYBRID CROSS: Phenotype Ratio: 9:3:3:1

RR WW RR Ww Rr WW Rr Ww

RR Ww RR ww Rr Ww Rr ww

Rr WW Rr Ww rr WW rr Ww

Rr Ww Rr ww Rr Ww Rr ww

RW Rw rW rwRW

Rw

rW

rw

5 TYPES OF GENETIC CROSSES The dihybrid cross is one of the 5 types

of genetic crosses we have studied. In a dihybrid cross, the inheritance of 2

traits is predicted at the same timeSince each parent has 4 possible

combinations of alleles, dihybrid crosses are 4 x 4 squares.

COMPLETE DOMINANCE Complete dominance is the first type we looked

at. One allele hides the effect of the other.

AA = dominant phenotype Aa = dominant phenotype aa = recessive phenotype

Possible crosses include: Heterozygous x heterozygous (Aa x Aa)

3:1 phenotype ratio, 1:2:1 genotype ratio Homozygous x heterozygous (aa x Aa and AA x aa)

1:1 phenotype and genotype ratios Homozygous x homozygous (AA x AA, aa x aa, and

AA x aa) 100% have the same genotype and phenotype

COMPLETE DOMINANCE EXAMPLE In humans, albinism

(a) is recessive to normal pigmentation (A). If an albino man has a child with a woman who is heterozygous for the trait, what is the probability their child will be an albino?

CODOMINANCE For codominant traits, BOTH alleles show up in

the phenotype AA = dominant phenotype Aa = both phenotypes (i.e. red + white fur) aa = recessive phenotype

Possible crosses include: Heterozygous x heterozygous (Aa x Aa)

1:2:1 phenotype ratio, 1:2:1 genotype ratio Homozygous x heterozygous (aa x Aa and AA x aa)

1:1 phenotype and genotype ratios Homozygous x homozygous (AA x AA, aa x aa, and

AA x aa) 100% have the same genotype and phenotype

CODOMINANCE EXAMPLE In a species of

tropical frog, there are frogs that have blue skin, frogs that have black skin, and frogs that have both black and blue skin.

If two black and blue frogs mate, what will the phenotype ratio of their offspring be?

INCOMPLETE DOMINANCE Traits with incomplete dominance produce a

heterozygous phenotype that is a blend of the two allele’s effect AA = dominant phenotype Aa = blended phenotype (pink flowers aa = recessive phenotype

Possible crosses include: Heterozygous x heterozygous (Aa x Aa)

1:2:1 phenotype ratio, 1:2:1 genotype ratio Homozygous x heterozygous (aa x Aa and AA x aa)

1:1 phenotype and genotype ratios Homozygous x homozygous (AA x AA, aa x aa, and

AA x aa) 100% have the same genotype and phenotype

INCOMPLETE DOMINANCE EXAMPLE A type of pepper

comes in three colors, red, orange, and yellow. If a gardener crosses two orange pepper plants, what will the phenotype ratio of the offspring be?

SEX-LINKED TRAITS The alleles for sex linked traits are

located on the sex chromosomes (X chromosome in humans)

Recessive sex-linked traits are much more common in males than females, since they have only 1 copy

SEX-LINKED TRAITS

In a Punnett square for a sex-linked trait, the sex chromosomes are used instead of letters.

X+X+ normal f X+Xo carrier f XoXo color-blind f X+Y normal m XoY color-blind m

X+ Xo

X+ X+X+

X+Xo

Y X+Y XoY

SEX-LINKED EXAMPLE Because she has a

brother with hemophilia, a biology student wanted to figure out the probability that she was a carrier for the X-linked disorder. Neither of her parents has hemophila. What is the probability she is a carrier of the trait?

QUIZ MONDAY! There will be a quiz on genetic crosses

this Monday! Each of the 5 types will show up on it.

Do your homework for practice!