Inheritance and Human Genetics

Sex Determination

• Use a punnett square to represent how sex is determined in offspring

X Y

X

X

X

X

Y

Y

X X

X X

23 Pairs of Chromosomes

Recessive X-linked Traits

• Occurs mostly in males• Colorblindness• Hemophilia– Impairs the ability of the blood

to clot after a cut, bruise, or other injury

• Duchenne Muscular Dystrophy (DMD)– Lack of skeletal muscle

Sex Linked Traits

• Normal eye sight - XN

• Color Blind - Xn

Xn Y

XN

Xn

XN

Xn

Y

Yn

X XN

Xn Xn

Single-Allele Traits

• Traits that are controlled by a single allele

• Dominant single-allele traits – Huntington’s disease– Some early symptoms of HD are

mood swings, depression, irritability or trouble driving, learning new things

– As the disease progresses, difficulty feeding himself or herself and swallowing

Single Allele trait

• Recessive single-allele trait

• Cystic Fibrosis– An inherited disease

that causes thick, stickymucus to build up in the lungs and digestive tract.

– Most people with cystic fibrosis died in their teens.

Single Allele trait

• Recessive single-allele trait• Sickle cell anemia

– Hemoglobin is a protein inside red blood cells that carries oxygen.

– Sickle cell anemia is caused by an abnormal type of hemoglobin called hemoglobin S.

– Hemoglobin S distorts the shape of red blood cells, especially when there is low oxygen.

Single Allele trait• Recessive single-allele trait• Phenylketonuria (PKU)– Accumulation of phenylalanine in

tissues. Lack of normal skin pigment and can cause mental retardation

• Tay-Sach’s Disease– Lipid accumulation in brain

cells(CNS), mental deficiency. Death in early childhood

Other types

• Multiple alleles (polygenetic)– ABO Blood Typing– Hair Color – Eye Color – Height – Skin Tone

• Pleiotropy occurs when a single gene influences multiple phenotypic traits.

Chromosomal Disorders

• Down syndrome – – Trisomy 21 or 3

copies of the 21st chromosome

• Turner’s syndrome – – One X chromosome,

usually called (45,X) Sterile

Chromosomal disorders

• Fragile X syndrome– Most

common mental impairment

• Klinefelter’s Syndrome – More than one X

chromosome – Sterile

Pedigree Analysis

Have you ever seen a family tree… do you have one??

Graphic representation of family inheritance.

Pedigree of Queen Victoria

What is a pedigree?• Shows a pattern of inheritance in a family for a

specific trait (phenotype)• Genotypes can usually be determined• Why would we want to use a pedigree in genetics?• Track the occurrence of diseases such as:

– Huntington’s – simple dominant – lethal allele – causes breakdown of the brain

– Cystic fibrosis – 1/2500 – mucus accumulates (white North Amer.)– Tay-Sachs disease – lipids accumulate in CNS (Jewish)– Phenylketonuria – missing enzyme causes problems in CNS

(Nordic/Swedish)

The Symbols used:

Sample pedigree:•generations are numbered with Roman Numerals•oldest offspring are on the left

How many males are present? How many females?How many females show the trait being studied?What is the sex of offspring III-9?How many offspring did the generation I parents have?What is the difference between the II-3 & 4 and IV-2 & 3?

Inheritance patterns:• Autosomal dominant:

–Examples: Polydactyly

–Huntington’s disease

The disease is passed from the father to the son, this never happens with X-linked traits.

The disease occurs in three consecutive generations, this never happens with recessive traits.

Males and females are affected, with roughly the same probability.

Inheritance patterns:• Autosomal recessive

– Cystic fibrosis– Tay-Sach’s disease

Males and females are equally likely to be affected.

The recurrence risk to the unborn sibling of an affected individual is 1/4.

The trait is characteristically found in siblings, not parents of affected or the offspring of affected.

Parents of affected children may be related. The rarer the trait in the general population, the more likely a consanguineous mating is involved.

Inheritance patterns:• Sex-linked recessive conditions

–Examples:

– Color-blindness

– Duchenne Muscular Dystrophy

The disease is never passed from father to son.

Males are much more likely to be affected than females.

• All affected males in a family are related through their mothers.

Trait or disease is typically passed from an affected grandfather, through his carrier daughters, to half of his grandsons.

ABO Blood Typing/ Pedigree

ABO Blood Typing/ Pedigree

• The genotype of individual I-1 is

A. iiB. IAiC. IBiD. IAIA

E. IAIB

Lets try and figure out what’s going on