Human Inheritance & Exceptions to the

Rule of Dominance & Recessiveness

Traits Controlled by Single Gene (Monohybrid)

• Many human traits are controlled by a single gene with one dominant allele and one recessive.

• Possess two very different phenotypes• Ex. Widow’s Peak or Straight Hairline

Dimples

Punnett Square for Single Gene with 2 Alleles

Multiple Alleles

• Multiple Alleles– A gene that has three or more alleles that

code for a single trait.• Human Blood Type

– Controlled by three alleles– IA, IB, i– IA and IB are codominant

Codominance

• Both dominant and recessive alleles are expressed (multiple alleles) in heterozygous individuals.

• Ex. Human Blood Type• Ex. #2 Erminette chicken feathers

– Both black and white feathers

Human Blood TypeBlood Types

Blood Type Combination of Alleles

A IAIA or IAi

B IBIB or IBi

AB IAIB

O ii

Codominance ProblemExample: homozygous male Type B (IBIB)

x heterozygous female Type A (IAi)

IAIB IAIB

IBi IBi

1/2 = IAIB

1/2 = IBi IA

IB IB

i

Another Codominance Problem

• Example:Example: male Type O (ii) x female type AB (IAIB)

IAi IBi

IAi IBi

1/2 = IAi1/2 = IBi

i

IA IB

i

CodominanceQuestion:

If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents?

boy - type O (ii) X girl - type AB (IAIB)

CodominanceAnswer:

IAIB

ii

Parents:genotypes = IAi and IBiphenotypes = A and B

IB

IA i

i

Incomplete DominanceHeterozygotes Heterozygotes have an appearance somewhat in in

betweenbetween the phenotypes phenotypes of the dominant and the recessive. (Mix)

Example:Example: snapdragons (flower)snapdragons (flower)red (RR) x white (rr)

RR = red flowerRR = red flowerrr = white flowerr

r

R R

Incomplete Dominance

RrRr

RrRr

RrRr

RrRr

rr

rr

RR RR

All Rr =All Rr = pink pink(heterozygous pink(heterozygous pink))

Incomplete Dominance

Traits Controlled by Many Genes

• Large number of phenotypes– Usually indicates that the trait is controlled

by many genes– Genes act together as a group to produce

a single trait.• Example

– Height & Skin Color

Effect of the Environment

• Effects of genes are often altered by the organism’s surroundings.

• Ex. Since the late 1800’s, the average height in the U.S. has increased by almost 10 cm.

Male of Female?

• Sex Chromosomes– 23rd pair of chromosomes– XX genotype=Female, XY genotype=Male

Sex-Linked Genes

• Sex-Linked Traits– Traits (genes) located on the sex chromosomes– Many sex-linked traits carried on X chromosome– Many genes/alleles are not found on male Y chromosome– Because males have only one X chromosome, males are

more likely than females to have a sex-linked trait that is controlled by a recessive allele.

• Carrier– A person who has one recessive allele for a trait and one

dominant allele. Only females in sex-linked traits.– Does not have the trait but can pass the recessive (disease)

allele to her offspring.

Red-Green Colorblindness• Sex-linked trait• A girl who receives only one recessive allele (written

XC) will not have the trait.• A boy who receives only one recessive allele will be

colorblind.

Sex-linked Traits

Sex ChromosomesSex Chromosomes

XX chromosome - female Xy chromosome - male

fruit flyeye color

Example: Example: Eye color in fruit Eye color in fruit fliesflies

Sex-linked Trait ProblemExample: Eye color in fruit flies (red-eyed male) x (white-eyed female)

XRY x XrXr

Remember: the Y chromosome in males does not carry traits.

RR = red eyedRr = red eyedrr = white eyedXy = maleXX = female

XR Xr

XR Xr

Xr y

Xr y

Xr

XR y

Xr50% red eyed female

50% white eyed male

Pedigrees

• If you were a geneticist trying to study human inheritance patterns, how would you go about doing that?

• You couldn’t set up crosses with people as Mendel did with peas.

• Need to trace the inheritance of traits through many generations in a number of families.

• Pedigree– Tool used to trace the inheritance of traits in humans– A chart of “family tree” that tracks which members have a

particular trait.

Reading a Pedigree

A half-shaded circle or square indicates that a person is a

carrier.

A completely shaded circle or square indicates that a

person has the trait.

A circle or square that is not shaded in green indicates that a person neither has the trait nor is a carrier.

A horizontal line connecting a male and female represents

a marriage.

A vertical line and a bracket connect the parents to their

children.

A circle represents a female.

A square represents a

male.

Female Carriers

Human Inheritance Summarysuch as

such as

such as

such as

controlled byHuman traits

Single genes

Multiple alleles

Many genes

Sex-linked genes

Widow’s peak

Blood type

Height

Colorblindness

Summary Questions

• If a gene has multiple alleles, why can a person only have two of the alleles for the gene?

• Why to human traits such as height and skin color have many different phenotypes?

• Explain why red-green colorblindness is more common in males than in females.

• What is a pedigree? How are pedigrees used?• Could two people with widow’s peak have a child with

a straight hairline? Could two people with strait hairlines have a child with a widow’s peak? Explain.