Chapter 15: Chromosomal Basis of Inheritance

What is the Chromosomal Theory of Inheritance?

• Gene have specific Loci on chromosomes

• Chromosomes will undergo segregation and independent assortment

Who discovered Chromosomal Linkage?

• Thomas Hunt Morgan by studying Drosophilia melanogaster fruit fly eye color

Sex linkage is

Gene located on a sex chromosomes

Linked genes

Genes located on the same chromosome that tend to be inherited together

What are the two classes of chromosomes?

Autosomal chromosomes (22)

Sex chromosomes

How did Morgan discovered Sex linkage

• Punnett’s Square of autosomal chromosomes

• Punnett’s Square of Sex chromosomes

• The results with the color of the fly eye matches the theory of sex linked inheritance

Genetic Recombination can occur through

• Crossing Over

• Genetic map

• Linkage map

Crossing over characteristics

Genes DO NOT assort independently of each other

Genetic Map Characteristics

The further apart 2 genes are, the higher probability that a cross over will occur between them= higher recombination frequency

Linkage map

Genetic map based on recombination frequency

Y chromosomes Characteristics

• Fewer genes other than SRY

• Sex determining region

• maleness

  • Male hormones

Pleiotropy

many effects

X Chromosomes Characteristics

• Other genes beyond sex determination

Human sex Linkage: SRY gene

Gene on Y chromosome that triggers the development of Testes

Human sex Linkage: Father

Only Pass X to daughters

Human sex Linkage: Mother

Pass X to both son and daughter

Human sex Linkage: Sex linked disorder

• Color-blindness

• Duchenne muscular dystropy (MD)

• hemophilia

( all recessive disorder)

X inactivation

• Inherit 2 X chromosoem

• One gets inactive during embryonic development

• condenses into a compacted body= Barr body - Random reaction

• Patchwork trait

What is an example of X inactivation

• Tricolor cats (only females)

• Tortoiseshell gene in cat

Chromosomal Abnormalities: Error in Meiosis- NONDISJUCTION

Incorrect number of Chromosomes

Chromosomes did not separate properly during Meiosis

Chromosomal Abnormalities: Error in Meiosis- BREAKAGE OF CHROMOSOMES

• Deletion

• Duplication

• Inversion

• Translocation

Nondisjunction cause

• Problem with meiotic spindle causing error with daughter cell

• Too many or too few chromosomes

Nondisjunction: Homologous Chromosomes

Did not separate properly during Meiosis 1

Nondisjunction: sister chromatids

Fail to separate during Meiosis 2

Aneuplody

Fetus (baby) with wrong Number of Chromosomes

Trisomy

Cell have 3 copy of chromosome

Monosomy

Cell have only one copy co chromosome

Which of the following about nondisjunction is true?

All of the above is correct

Human Chromosome Disorfer

• Most Embryo are spontaneous aborted

• If born, developmental problems by biochemical imbalance

• Some are survival

Syndrome

Characteristics set of symptoms

Down Syndrome

• Trisomy of 21 (3 copy on chromosome 21)

• 1 in 700 children in US

• 21 is the smallest chromosome

• correlates with the age of the mother

Klinefelter’s Syndrome male

• XXY

• 1/2000 birth live

• Male sex organ but sterile

• some female characteristics: breast and lack of hair

• Tall

• Normal Intelligence

Jacob’s Syndrome Male

• XYY

• 1/1000 birth live

• Extra Y chromosome

• slightly taller than average

• more active

• normal intelligence, slight learning disabilities

• delayed emotional maturity

• normal sexual development

Trisomy X

• XXX

• 1/2000 live

• Healthy female due to Barr body

Turner Syndome

• Monosomy X or X0

• 1/5000 birth

• varied degree of effects

• webbed neck

• Short statue

• Sterile

Changes in chromosomal structure: Deletion

Loss of chromosomal segment

Changes in chromosomal structure: Duplication

repeated segment

Changes in chromosomal structure: Inverson

Reverse a segement

Changes in chromosomal structure: Translocation

Move segment form one chromosome to another

Genomic Imprinting Definition

Parental Effect on gene expression

• Identical allele might have different effect on offspring depending if it arrives from sperm or ovum

Fragile X syndrome

Higher prevalence of disorder and retardation in males

Which genetic condition is caused by multiple X chromosomes being present but only one X chromosome is inactivated?

Trisomy X