Patterns of Heredity and Human Genetics

Simple Dominant Heredity

This type of heredity is what Mendel observed.

It only takes one dominant allele for an organism to show a dominant trait.

For example, the genotypes RR and Rr would show the same phenotype of ROUND seeds.

Simple dominant traits

Tongue rolling Hapsburg lip (protruding lower lip) Free earlobes Hitchhiker’s thumb Almond shaped eyes Thick lips Presence of hair on middle knuckles.

Incomplete dominance

Phenotype of the heterozygote is intermediate between those of 2 homozygotes. Example: Homozygous red flower (RR) is

crossed with a homozygous white-flowered plant (R’R’), all the offspring will have pink flowers.

Neither allele of the pair is completely dominant.

Incomplete dominance

RR’ RR’

RR’ RR’

R R

R’

R’

RR RR’

RR’ R’R’

R R’

R

R’

Why does this happen?

R allele codes for an enzyme that produces red pigment

R’ allele codes for a defective enzyme that makes no pigment

If the genotype is RR’ it only makes half the pigment thus causing the phenotype to be pink.

Codominance occurs when both alleles for a gene are expressed in a heterozygous offspring.

In Codominance, neither allele is dominant or recessive, nor do the phenotypes appear to blend. Both alleles of a gene are active and influence the phenotype.

Codominant genes are written as capital letters with a different letter for each phenotype. Cows demonstrate codominance in regards to hair color.

R and WRR = RedWW = WhiteRW = Roan (both red and white)

Example: White Cow (WW) x Red Bull (RR)

Results: Genotype: RWPhenotype: Roan

R R

W

W

RW RW

RW RW

* Pay attention to the ‘ it does not matter if it is on the 1st or 2nd letter

Codominance in humans Sickle cell anemia

Most common in Americans whose families originated from Africa

1 in 12 African Americans is heterozygous for the disorder.

An individual who is homozygous for the sickle-cell allele, the oxygen-carrying protein (hemoglobin) differs by one amino acid from normal hemoglobin.

Sickle-cell anemia The defective hemoglobin forms crystal-like

structures that change the shape of red blood cells. They are shaped like a sickle (half moon) This shape causes slow blood flow, blocked small

vessels and tissue damage

Polygenic inheritance

Traits such as skin color and height vary over a wide range.

These wide ranges occur because these traits are governed by many different genes.

Polygenic inheritance is the inheritance pattern of a trait that is controlled by 2 or more genes.

Multiple phenotypes from multiple allelesTraits controlled by more than two alleles in a

population have multiple allelesBlood type is an example of a single gene that has multiple alleles in humans.

Alleles for blood type:

IA

IB

i

Multiple alleles in humans

Sex determination Humans have 23 pairs of chromosomes. 22 of these pairs are autosomal (matching

homologous chromosomes) Homologous autosomes look exactly alike. The 23rd pair differs in males and females. These are sex chromosomes.

Sex-linked inheritance

Traits controlled by genes located on sex chromosomes are called sex-linked.

Alleles for sex-linked traits are written as subscripts of the X or Y chromosome.

X and Y chromosomes are not homologous, therefore the Y chromosome has no corresponding allele on the X chromosome and no subscript is used. Any allele on the x chromosome of a male will not be

masked by a corresponding allele on the Y chromosome!

Sex linked traits in humans

Sex linked traits are inherited on the sex chromosomes Most are located on the X chromosome Males pass an X chromosome to their daughters and a Y

chromosome to their sons Females pass an X to both

If a son receives an X chromosome with a recessive allele from his mother, he will express the trait because there is no chance of inheriting a dominant allele from his father to mask the trait (X and Y are not homologous)

XX

XcYXY

XXc

X Xc

X

Y

Example: colorblindness

Each male child whose mother is a carrier for a defect has a 50% chance of inheriting the defect

Each female child whose mother is a carrier for a defect has a 50% chance of becoming a carrier

Red-green color blindness Color blindness is caused by the inheritance of

either of 2 recessive alleles at 2 gene sites on the X chromosome that affect the red and green receptors in the cells of eyes

Hemophilia Inability to clot blood X-linked disorder; affects 1 in every 10,000

males Only affects 1 in 100 million females

Males inherit the allele on the X chromosome from carrier mothers

A single recessive allele will cause the disorder in males

Females need 2 recessive alleles to inherit hemophilia Queen Victoria’s family is the most well-known

study for hemophilia

Sex-Influenced Traits

The presence of male or female sex hormones influences the expression of certain human traits. Estrogen Testosterone

With this type of trait, males and females have different phenotypes even when they have the same genotype.

These genes are located on autosomes.

Example: Pattern Baldness

B = dominant, hair loss B’ = normal, no hair loss BB male = hair loss BB female = hair loss BB’ male = hair loss BB’ female = NO hair loss The differences in gene expression are due to

higher levels of testosterone in men.