Recessive genetic disorders and inheritance patterns

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RECESSIVE DISORDERS

Simple Recessive Disorders

Recessive Disorders affect offspring with 2 recessive alleles (homozygous recessive)

Most genetic disorders are caused by recessive alleles

Cystic Fibrosis

1 in 35 white Americans carry the recessive allele

Defective PROTEIN in the plasma membrane

Accumulate mucus in the lungs and digestive tract

Tay Sachs

Absence of ENZYME that breaks down LIPIDS

Lipids accumulate in cells

Phenylketonuria (PKU)

Absence of ENZYME to convert phenylalanine to tyrosin

Phenylalanine cannot be broken down

Accumulate in cells

Damage CNS

Mental Disabilities and cognitive delays

INCOMPLETE DOMINANCE

Heterozygote is a mix

A third phenotype appears

Flowers contain enzymes that control pigment production

The R allele makes a red pigment

R defective enzyme that makes no pigment

RR plenty red pigment= Red

RR has one R allele and can produce half the amount of red pigment= PINK

RR can produce no pigment= White

CODOMINANCE

Heterozygote has both alleles

USe BOTH uppercase letters

Both alleles show equally

SICKLE-CELL ANEMIA

(CODOMINANCE)

Hemoglobin, the oxygen-carrying protein, differs by one amino acid, which changes the shape of red blood cell

Abnormal shape slows blood flow and blocks small vessels

Sickled cells have a shorter life span

Anemia (low # of red blood cells)

Causes tissue damage

Patients may have painful attacks

Individuals who are heterozygous for the allele produce both normal and sickled red blood cells (codominance)

Able to produce enough hemoglobin to have no serious health problems

"have the sickle-cell trait"

Why does the sickle cell allele persist?

Carriers (with one sickle gene and one normal hemoglobin gene) have some protective advantage against malaria

The frequencies of sickle cell carriers are high where malaria is prevalent.

People who have sickle cell anemia are less likely to die from malaria. The parasite is carried by the Anopheles mosquito. Cells that sickle makes a poor home for this parasite spreads too widely before the person gets seriously ill.

Multiple Alleles

Traits controlled by 2 or more alleles

Pigeon:

Ash red (Ba)=dominant to all

Blue (B)= dominant to b, but recessive to Ba

Chocolate (b)= recessive to all

Blood type & multiple alleles

ABO- Human blood groups

I is a gene for blood type

I code for a molecule that attaches to a membrane protein on the surface of red blood cells

Each allele codes for a different surface molecule

Blood groups:

A, B, AB, and O

3 alleles:

IA, IB, I

Genotype: IAIA and IAi, IBIB and IBi, IAIB, ii

Surface molecule: A, B, A & B, none

Phenotype: A, B, AB, O

Incompatible

Sex-linked inheritance

Humans have 23 pairs of chromosomes (46 chromosomes in total)

22 pairs of autosomes 1 pair of sex chromosomes

Females: sex chromosomes are XX

Males: Sex chromosomes are XY

Male determines sex offspring

At fertilization, sex is determined by laws of probability.

"Nipples are the buttons that hold everything together" -Trinity Hayes, 1/27/20

The X and Y chromosomes are not homologous- the Y chromosome has no corresponding alleles to those on the X chromosome.

For males, recessive alleles on the X chromosome will NOT have another allele to "Mask it on the Y

Traits are controlled by genes on sex chromosomes

Colorblindness: An X-linked disorder

lack of one or more color receptors

Rods- night vision; cones-pigments

Usually genetic

No treatment, but most people adjust

Condition in which blood does not properly clot

1/10,000 males

1/1,000,000 females

Males inherit the allele for hemophilia on the X chromosome from mother

A single recessive allele will cause the disease

(males would need 2 recessive alleles)

Trait is controlled by more than one gene

All heterozygotes are intermediate phenotype

Results in range of phenotypes

Polygenic Inheritance:

Skin color

Eye color

Height and intelligence