Patterns of Heredity and Human Genetics

Variation in 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.

aka. Complete Dominance

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.

Sickle-cell anemia An individual who is homozygous for the sickle-cell

allele, the oxygen-carrying protein (hemoglobin) differs by one amino acid from normal hemoglobin.

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.

IA, IB, i code for blood type

Genotype Blood type IA IA (homozygous) A IA i (heterozygous) A IB IB (homozygous) B IB i (heterozygous) B IA IB (codominance) AB i i (homozygous) O

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 (allosomes).

Sex-linked inheritance Traits controlled by genes located on sex

chromosomes are called sex-linked. Alleles for sex-linked traits are written as

superscripts 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 superscript 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)

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

XX

XcYXY

XXc

X Xc

X

Y

Example: 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

HemophiliaInability to clot bloodX-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 = normal, dominant, no hair loss Bi = recessive, hair loss (Baldness) BiBi male = hair loss (Baldness) BiBi female = hair loss (Baldness) BBi male = hair loss (Baldness) BBi female = NO hair loss The differences in gene expression are due

to higher levels of testosterone in men.