HUMAN GENETICSHUMAN GENETICS

HUMAN GENETICSHUMAN GENETICS

• 1. WHAT ARE SOME EXAMPLES IN HUMANS OF CHROMOSOMAL EFFECTS?

• 2. WHAT ARE SOME EXAMPLES IN HUMANS OF AUTOSOMAL DOMINANT AND RECESSIVE TRAITS?

HUMAN GENETICSHUMAN GENETICS

• 3. WHAT ARE SOME OTHER FORMS OF INHERITANCE IN HUMANS?

• 4. WHAT IS A PEDIGREE?

• 5. HOW CAN GENETIC DISORDERS BE DETECTED?

1. WHAT ARE SOME 1. WHAT ARE SOME EXAMPLES IN HUMANS OF EXAMPLES IN HUMANS OF CHROMOSOMAL EFFECTS?CHROMOSOMAL EFFECTS?

Chromosomal InheritanceChromosomal Inheritance

All but one pair of chromosomes in males and females are the same.– Autosomes - Nonsex chromosomes

The different pair, sex chromosomes, determines the sex of an individual.– X-linked (sex-linked) is the term used for genes

carried on the X chromosome.

X-Linked AllelesX-Linked Alleles

X-linked alleles have a different pattern of inheritance than alleles on autosomes because the Y chromosome is blank for these alleles.– Inheritance of a Y chromosome cannot offset

the inheritance of an X-linked recessive allele.

Human X-Linked DisordersHuman X-Linked Disorders

Color Blindness– In humans, color vision receptors in the retina

are three different classes of cone cells. Only one type of pigment is present in each class of

cone cell.– The allele for blue-sensitive is autosomal, but the red- and

green-sensitive proteins are on the X chromosome.

Human X-Linked DisordersHuman X-Linked Disorders

Muscular Dystrophy– Absence of protein dystrophin allows calcium

to leak into muscle cells.

Hemophilia– Hemophilia A due to lack of clotting factor IX

and hemophilia B due to lack of clotting factor VIII.

Changes in Chromosome Changes in Chromosome NumberNumber

Monosomy and Trisomy– Monosomy (2n - 1) occurs when an individual

has only one of a particular type of chromosome.

– Trisomy (2n + 1) occurs when an individual has three of a particular type of chromosome.

Changes in Sex Chromosome Changes in Sex Chromosome NumberNumber

An abnormal sex chromosome number is the result of inheriting too many or too few X or Y chromosomes.– Nondisjunction during oogenesis or

spermatogenesis.

TURNER SYNDROME X0

TURNER SYNDROME XO KLINEFELTER XXY

Sex Chromosome SyndromesSex Chromosome Syndromes

Poly-X Females– More than two X chromosomes and extra Barr

bodies in the nucleus. Range from tall and thin to tall and severely retarded

depending on number of X chromosomes.

Jacobs Syndrome– XXY due to nondisjunction during spermatogenesis.

Taller than average

Changes in Chromosome Changes in Chromosome StructureStructure

Deletion– End of a chromosome breaks off, or two

simultaneous breaks lead to loss of an internal segment.

Translocation– Movement of a chromosome segment from one

chromosome to another, non-homologous chromosome.

WILLIAMS SYNDROMEWILLIAMS SYNDROME

ALAGILLE SYNDROME

Changes in Chromosome Changes in Chromosome StructureStructure

Duplication– Presence of chromosomal segment more than

once in the same chromosome. Known to occur as a result of an inversion in which

segment is turned 180o.

2. WHAT ARE SOME 2. WHAT ARE SOME EXAMPLES IN HUMANS OF EXAMPLES IN HUMANS OF AUTOSOMAL DOMINANT AUTOSOMAL DOMINANT

AND RECESSIVE TRAITS?AND RECESSIVE TRAITS?

Human Genetic DisordersHuman Genetic DisordersAutosome - Any chromosome other than a sex chromosome.– When a genetic disorder is autosomal dominant, an

individual with AA or Aa has the disorder.– When a genetic disorder is autosomal recessive, only aa individuals have the disorder.

Carriers - Individuals unaffected by a disorder but can have an affected child.

Human Genetic DisordersHuman Genetic DisordersAutosome - Any chromosome other than a sex chromosome.– When a genetic disorder is autosomal dominant, an

individual with AA or Aa has the disorder.– When a genetic disorder is autosomal recessive, only aa individuals have the disorder.

Carriers - Individuals unaffected by a disorder but can have an affected child.

Autosomal Recessive Autosomal Recessive DisordersDisorders

Tay-Sachs Disease– Progressive deterioration of psychomotor functions.

Cystic Fibrosis– Mucus in bronchial tubes and pancreatic ducts is

particularly thick and viscous.

Phenylketonuria– Lack enzyme for normal metabolism of phenylalanine.

Autosomal Dominant Autosomal Dominant DisordersDisorders

Neurofibromatosis– Tan or dark spots develop on skin and darken.

Small, benign tumors may arise from fibrous nerve coverings.

Huntington Disease– Neurological disorder leading to progressive

degeneration of brain cells, in turn causing severe muscle spasms and personality disorders.

3. WHAT ARE SOME 3. WHAT ARE SOME OTHER FORMS OF OTHER FORMS OF

INHERITANCE IN HUMANS?INHERITANCE IN HUMANS?

Multiple Allelic TraitsMultiple Allelic Traits

When a trait is controlled by multiple alleles, the gene exists in several allelic forms.– ABO blood types

Phenotype Genotype

A IAIA,IAi

B IBIB,IBi

AB IAIB

O ii

LARGE NUMBER OF TRIPLET REPEATS OF CGG AT FRAGILE SITE ON X CHROMOSOME

4. WHAT IS A PEDIGREE?4. WHAT IS A PEDIGREE?

5. HOW CAN GENETIC 5. HOW CAN GENETIC DISORDERS BE DISORDERS BE

DETECTED?DETECTED?