Chapter 12 CHROMOSOMES

A. What is a Chromosome? A long, continuous strand of DNA, plus several types of associated proteins, and RNA.

Chromosome pairs are distinguished by: size, banding pattern & centromere position.

B. Linked Genes Genes carried on the same chromosome. Linked genes violate Mendel’s law of independent assortment because they may not separate during crossing-over of meiosis I.

What types of gametes are expected from this individual? ¼ PL ¼ pl ¼ Pl ¼ pL What types of gametes are expected from this individual? ½ PL ½ pl

The further apart two linked genes are, the more likely they will separate during gamete formation.

Parental gametes retain the gene combinations from the parents. Recombinant gametes result from the mixing of maternal & paternal alleles during crossing- over. Closely linked genes yield few recombinant chromosomes - will NOT obtain expected 9:3:3:1 phenotypic ratio.

Knowing allele arrangement is important in predicting trait transmission. Ex. Two allele combinations are possible for a pea plant with genotype PpLl. Alleles in cis tend to be transmitted together (P, L 은 같이 전달됨 : 우성끼리 ; 열성에 대해서도 같은 방법으로 ). Alleles in trans separate with each generation (P, L 은 서로 분리되어 전달됨 ; 열성에 대해서도 같은 방법으로 ).

C. Sex Determination Mechanism by which an individual develops as a male or a female. 1. Total chromosome number female is diploid (develops from a fertilized ovum) male is haploid (develops from an unfertilized ovum) Ex. bees

2. X-O System (number of X chromosomes determines sex) female is XX male is XO Ex. grasshoppers, crickets & roaches 3. X-Y System (presence of Y chromosome determines sex) SRY gene female is XX male is XY Ex. all mammals

In X-Y system, male determines sex of offspring.

D. Inheritance of Genes on Sex Chromosomes X-Linked traits are carried on the X chromosome and most are recessive Examples: color blindness, hemophilia More common in females; affected males are hemizygous, cannot be carriers Male inherits X-linked condition from carrier or affected mother Y-linked traits are very rare because Y has few genes

Hemophilia: recessive X-linked trait GenotypePhenotype X H X H non-carrier female X H X h carrier female X h X h female with hemophilia X H Ynormal male X h Ymale with hemophilia

What is the probability that a carrier female and a normal male will have a son with hemophilia? ¼ or 25% What is the probability that a non- carrier female and a hemophiliac male will have a son with hemophilia?

E. X Inactivation Female mammals have 2 alleles for every gene on the X chromosome, while males have only 1. This inequality is balanced by “turning off” one X chromosome in each cell of a 3 week old female embryo. some cells turn off paternal X some cells turn off maternal X

Inactivated X appears as a dark- staining structure called a Barr body. How many Barr bodies would cells of a male possess?

X inactivation is responsible for the appearance of calico cats. The earlier X inactivation occurs, the larger the patches.

F. Chromosome Abnormalities 1. Polyploidy - extra full sets of chromosomes. animal polyploids spontaneously abort or die shortly after birth plant polyploids are relatively common (wheat, lilies)

2. Aneuploidy - an extra (trisomy) or missing (monosomy) chromosome. Aneuploidy is usually due to a meiotic error called nondisjunction.

Autosomal aneuploids trisomy 13 trisomy 18 trisomy 21 (Down syndrome) Sex chromosome aneuploids Turner syndromeXO female Triplo-X XXX female Klinefelter syndromeXXY male Jacobs syndromeXYY male

3. Deletion - part of a chromosome is missing. 4. Duplication - part of a chromosome is present twice. 5. Inversion - part of a chromosome is reversed.

6. Translocation - nonhomologous chromosomes exchange parts (reciprocal translocation) or combine (Robertsonian translocation).