Chromosomal Genetic

Why are some traits inherited in ways other than Mendel’s?

Linked genesGene (chromosome) mapping

Linked genes

Morgan: studied eye color in Drosophila (flies)P1 true breeding white eyed males mated withP1 true breeding red eye femalesF1: all red eyes (expected)F2: 3:1 red eyes to white (expected)BUT, the white eyes showed up only in MALESAll females had red eyes: about half the males

had red…..the other half of the males had whiteCONCLUSION: eye color linked to sex of fly

Phenotypes

In a cross: the offspring will get either: Parental phenotype: same as 1 of the parents

OR

recombinant phenotype NOT same as either parent

Genetic Recombination

Recombination of unlinked genes: Independent assortment of chromosomes

Recombination of Linked genes: Crossing over

Linkage maps

Linkage maps are based on the frequency of recomination.

If 50% recombo same as if on separate chromosomes

1% crossing over = 1 map unit

When homologous chromosomes pair and crossing over occurs, new gene linkages can result. Genes that are far apart on the same chromosome should be separated more often than genes that are close together. Morgan was able to calculate mathematically how close or far apart each particular gene pair seemed to be based on the frequency of crossing over. map?

Example: Genes A, B, C, and D are on the same chromosome. The following represent the frequency with which these genes

are separated during crossing over:A – B 8%A- C 28%A- D 25%B- C 20%B-D 33%What is the order of these genes on a

chromosome

D – A – B - C

Linkage Map Problems

Draw a linkage map(s) for the following recombination frequencies:

 1. A-B 30%, B-C 20%, B-D 35%, C-D 25%, A-C

50%, A-D 50% 2. A-C 50%, C-D 15%, A-D 50%, A-B 30%, B-C

50%, B-D 50%

Sex linked recessive traits are more common in MALES

Sex linked disorders (recessive)

SEX LINKED DISORDERSRed green color blindness- malfunction in

light sensitive cells in the eyeHemophilia- lack one or more proteins

needed for blood clottingDuchenne muscular dystrophy- progressive

weakening of muscles and loss of coordination

Sex linked recessive

Common Chromosomal Abnormalities

http://anthro.palomar.edu/abnormal/abnormal_4.htm

Two Main Categories Structural modification Irregular number

Typically result from nondisjunction during meiosis

Polyploidy-complete multiples of sets of chromosomes (23+23+23)

Aneulploidy-addition or loss of chromosomes within a set (23+22)

Autosomal Defects

http://anthro.palomar.edu/abnormal/abnormal_4.htm

Down’s SyndromeTypically trisomy of

chromosome 21Some have

translocation of 21 to 14 or 15

2-4% are genetically mosaic

75-80% of Down syndrome children are born to women under 35

MonosomyOnly one set of

chromosomes remains after fertilization

All fetuses will spontaneously abort

Female AbnormalitiesFemale Abnormalities Male AbnormalitiesMale Abnormalities

http://anthro.palomar.edu/abnormal/abnormal_4.htm

Metafemale(XXX) Unusually tall, low to

normal intelligence Normal sex

characteristics, fertileTurner Syndrome

(XO) Ovaries do not develop,

and do not ovulate Lack secondary sex

characteristics Slight mental retardation

Klinefelter Syndrome (XXY) High-pitched voice Asexual to feminine body

type Low testosterone, sterile 1 in 500

XYY Syndrome Unusually tall, severe acne High testosterone levels,

possibly leading to violence

Sex Chromosome Abnormalities