HeredityChapter 12, part 2

Beyond Mendel’s Laws• Not all traits are controlled by single

genes with dominant and recessive alleles. Other patterns of heredity involve:

• Incomplete dominance

• Sex-linked traits

• Multiple alleles

• Multiple genes (most traits involve this)

• Chromosomal abnormalities

Incomplete Dominance

• In incomplete dominance and co-dominance, both alleles are expressed in the phenotype.

• incomplete dominance: two traits appear to blend in the heterozygotes.

• co-dominance: both traits appear in the heterozygotes.

These snapdragons

have two alleles controlling flower color: R (red) and

R’ (white). Heterozygotes (RR’) have pink flowers. Unlike

the case in complete

dominance, the phenotypes show us which plants

are heterozygous.

sper

m

eggs

mother

father

C2C1

C2C1

C2C1 C2

C1

C1C1 C1C2

C1C2 C2C2

In humans, a gene affecting

hair texture (curly, wavy,

straight) shows incomplete dominance.

The golden palomino horse is a cross between a white and a brown

horse. This is another example

of incomplete dominance: the colors appear to

blend in the horse’s hairs.

The red roan horse has both white and red-brown hairs, while the blue roan has both white and gray hairs. The coat colors of both parents are expressed in the hairs. This is co-

dominance.

At the gene level, incomplete dominance and co-dominance are

the same: in both cases, both alleles are expressed in the

heterozygote. The only difference is at the phenotype level.

Solving single-gene (monohybrid) crosses with incomplete dominance.

One hair color in cattle is controlled by a gene that produces red (R) or white (R’) hairs. Heterozygotes (RR’) are roan.a. What color would the offspring of a red bull and a white cow be?b. What are the phenotypic ratios of a cross between a white cow and a roan bull?

Solving single-gene (monohybrid) crosses with incomplete dominance.

One hair color in cattle is controlled by a gene that produces red (R) or white (R’) hairs. Heterozygotes (RR’) are roan.a. What color would the offspring of a red bull and a white cow be?b. What are the phenotypic ratios of a cross between a white cow and a roan bull?

Sex-Linked Traits• In humans, women have two X

chromosomes while men have an X and a Y.

• Traits that are carried on the sex chromosomes will show different genotypic and phenotypic rations in men and women.

• The X chromosome has many genes, while the Y has only a few, so there are many more X-linked traits than Y-linked traits.

Women pass their X chromosomes to their

children. Men can contribute either an X or a

Y.

Which parent determines the sex of the child? Mother or

father?Can men be carriers of

a recessive X-linked trait?

If a boy has an X-linked trait, which parent did

he inherit the trait from?

sper

m

X2X1

Y

Xm

female parent

male parent

female offspring

eggs

Y

Xm

XmX1

YX1 YX2

XmX2

When determining the outcome of a

cross that involves an X-linked trait, we have to take into account how

the two sex chromosomes are distributed in the

offspring.

sper

m

female parent

male parent

female offspring

eggs

XrXR

rR

XrXR XrXR

rR

XR

R

XRYYXR Y

R R

R

r R

r

XR XR Xr XR

all the F2

femaleshave red eyes

half the F2

males havered eyes, halfhave white eyes

This diagram illustrates a cross

for an X-linked trait in fruit flies. Red

eyes are dominant, white are recessive.

Red-green color blindness is an X-linked trait.

Normal RG color-blind

A 29 70

B 45 nothing

C abstract 5

D 26 nothing

Charts such as these are used to diagnose red-green color blindness. They look very different for those with normal vision and those with RG-

color blindness.

Solving X-linked crosses.

Red-green color-blindness is X-linked. If a man and woman with normal vision have a color-blind son:a. What are the genotypes of the parents?b. What are the odds of having another color-blind son?c. What are the odds of having a color-blind daughter?

Solving X-linked crosses.

Red-green color-blindness is X-linked. If a man and woman with normal vision have a color-blind son:a. What are the genotypes of the parents?b. What are the odds of having another color-blind son?c. What are the odds of having a color-blind daughter?

Multiple Alleles

• Human blood type (A, B, AB, and O) is determined by a gene that has three alleles.

• A and B are co-dominant

• O is recessive to both

• Though there are three alleles, each person still has only two copies of the ABO gene.

A and B alleles produce A and B proteins on the surface of red blood cells. The O allele

produces neither of these proteins.

Multiple Genes

• Most human traits are the result of multiple genes.

• In some cases (such as skin and hair color), there are multiple copies of the same gene (such as the melanin gene).

• In many others, there are many different genes controlling a trait, and the environment may affect how a trait is expressed. (Example: human height)

sperm

eggs

Human skin color is controlled by at

least three melanin-producing genes, which are

incompletely dominant.

This massive Punnet square shows a cross between two people

who are heterozygous for all

three genes.

Chromosomal Abnormalities

• Chromosomal abnormalities include:

• nondisjunctions (failure of chromatids to separate during meiosis)

• deletions of parts of chromosomes

• Most chromosomal abnormalities cause cell death, but a few are survivable.

Cri-du-chat syndrome is caused by a deletion of a

large segment of chromosome 5.

Cri-du-chat children often have small head

circumference and are severely cognitively

challenged. Some may have heart defects, muscular or skeletal problems, or vision

problems.

Trisomy 21 results in Down Syndrome.

Using what you know about meiosis, explain how a fertilized human

egg cell can end up with three copies of

chromosome 21.

If a person with Down Syndrome planned to have a child, could the

child inherit Down Syndrome?

Nondisjunction of the sex chromosomes is more often survivable than nondisjunctions of somatic chromosomes. As long as the fetus

has at least one X chromosome, it can survive.

Klinefelter syndrome produces an XXY male.

At puberty, Klinefelter males fail to fully

develop secondary sex characteristics. Men

with this syndrome may or may not be sterile. They have a tendency to gain weight easily

and their muscle mass is underdeveloped, but

mental function is usually normal.

Turner syndrome occurs when a girl inherits only one

X chromosome.

Turner children are often short, and may show

swelling in the hands and feet. Some have heart defects, but most are cognitively normal. Hormone therapy at

puberty can help Turner girls grow to normal height and develop secondary sex

characteristics.

• Traits inherited by classic Mendelian genetics are by far in the minority.

• Incomplete dominance, sex linkage, and multiple alleles involve single-gene traits that show unique patterns of inheritance.

• Most traits involve multiple genes and gene-environment interactions.

Recap