Chapter 15: Chromosomal Basis of Inheritance

(Link between Meiosis or cellular processes and Genetics)

Objectives

1. Mendelian Genetics is based on the physical behavior of chromosomes

2. Sex linked genes have unique inheritance patterns

3. Linked genes are often inherited together due to physical proximity on the chromosome

4. Alterations of chromosome number or structure results in genetic disorders

5. There are some exceptions to the inheritance patterns

Chromosome Theory

Chromosomes are what segregates and undergo independent assortment.

Chromosomal Behavior determines sex of organism

-There is a sex determining region on the Y chromosome.

-Gonads are generic until about 2 months

-SRY gene is responsible for the formation of the testes

Sex Linked Genes

-Sex linked recessive genes for color blindness

-homozygous recessive females have the trait

-males that get the recessive allele from mom will have the disease

X Chromosome Inactivation-One of the X chromosomes in females becomes in active.

-Barr Bodies are the inactive version of the X chromosome

-Inactivation doesn’t happen right away

-Females that are heterozygous for a certain X chromosome trait can express both traits during development.

-Inactive versions produce the same in active versions during mitosis.

Linked Genes and

Recombinant Chromosomes

-Parental types vs Recombinants p. 294

-Recombinations can happen when chromosomes segregate indpendently or after crossing over has occurred

-Crossing over is responsible for recombinant genes if they are located on the same chromosome

Location of genes on a chromosome

Chromosomal Dissorders-Nondisjunction – homologous chromosomes don’t move apart

-Aneuploidy – when a zygote has an abnormal chromosome amount

-monosomic one less

-trisomic one extra (polyploidy)

-Downs Syndrome (trisomy 21) – the older the woman is the greater the odds of having a baby with Downs

-Sex Chromosomes dissorders (Most are normal)

Klinefelters syndrome XXY – male is sterile

Turnner syndrome XO only known viable monosomy in humans – Females are sterile

*Barr Bodies are reason success of individuals

Alterations of Chromosome Structures*Many cases cause severe problems – most don’t make it to birth