Chapter 12: The Chromosomal Basis of Inheritance

12.1 Mendelian inheritance has its physical basis in the behavior of chromosomes

• Chromosomes & genes present in pairs - diploid cells. • Homologous chromosomes separate & alleles segregate during meiosis. • Fertilization restores pairs. • Chromosome Theory of Inheritance: Genes discovered by Mendelian have separate loci,

chromosomes undergo segregation & assortment. Morgan’s Choice of Experimental Organism: • Thomas Hunt Morgan found solid evidence for specific traits corresponding for specific

chromosomes. • Morgan used fruit flies as prolific breeders and only containing three pairs of autosomes and

one pair of sex chromosomes. • He discovered fly with white eyes instead of red- wild type. • Wild Type: Gene variation commonly found in wild genes. • Mutant Phenotypes: Variations caused by changes/mutations in alleles (white eyes). Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair: • Morgan discovered upon breeding that the wild type was dominant. • White eyes only showed in males - concluded eye color and sex were related. • Determined white eyes was on x gene of sex chromosome because with only one x, no

recessive wild type to bump white eyes. • No white eyes in females because father needed them and they only had red eyes (White eyes

appeared in F2)

12.2 Sex-linked genes exhibit unique patterns of inheritance.

The Chromosomal Basis of Sex • Two sex chromosome types exists - X&Y • Females contain XX and male XY • Only short sections of the Y are homologous with X. • Eggs get X, sperm get X or Y. • Zygote becomes male or female based on sperm. • Sex - linked Genes: genes located on sex chromosomes, they’re classified as X or Y linked

genes. Inheritance of X-Linked Genes • Fathers pass X-Genes to daughters but not sons, mothers can pass X to both. • Men cannot use homo or heterozygous, so genes that are similar are classified hemizygous. • More men have X-Linked genes than women, because any recessive alleles from mother are

expressed. • Color Blindness and Hemophilia are X-Linked Genes.

X Inactivation in Female Mammals • Most of one X chromosome is inactive after embryonic development in females. • Barr Body: the inactive X chromosome lies within the nuclear envelope with no use. • Within the ovaries barr bodies are activated leading to each female gamete having an active x. • Which x chromosome becomes the barr body is random. • Inactivation occurs when methyl groups attach to the nitrogenous bases in nucleotides. • Caused by XIST (X-inactive specific transcript) - only active on barr body X.

12.3 Linked genes tend to be inherited together because they are located near each other on the same chromosome.

• Linked genes: genes located near each other on the chromosome that are inherited together.

How Linkage Affects • Genetic recombination: Offspring with combinations not found in the parent generation. Genetic Recombination and Linkage • Parental types: Offspring that have phenotypes matching their parents. • Recombinant Types: Offspring with phenotypes not like parents. Mapping the Distance Between Genes Using Recombination Data: Scientific Inquiry • Genetic Map: Ordered list of loci on chromosome. • The farther apart two genes are the the higher the probably they will be crossed. • Linkage Map: Genetic map based on linkage frequencies. • Map Units: 1% recombination frequency. • Cytogenetic Maps: locates genes with help of stains and dyes.

12.4 Alterations of chromosomes number or structure cause some genetic disorders.

Abnormal Chromosome Number • Nondisjunction: Sister chromatids fail to move apart during meiosis I causing abnormal

chromosomal distribution during division. • Aneuploidy: Abnormal number of chromosomes in zygote due to nondisjunction. • Monosomic: When the zygote is missing chromatic information. • Trisomic: When three chromosomes are contained within the zygote due to nondisjunction

(Example: Down’s Syndrome). • Polyploidy: Containing more than two chromosomes in somatic cells. • Polyploidy is common in plants (ex: bananas - 3n, wheat - 6n, strawberries - 8n). Alterations of Chromosome Structure • Deletion: When chromosomal fragments are lost. • Duplication: Deleted fragment may attach to sister chromatid. • Inversion: Fragment that breaks off and reattaches in reverse place. • Translocation: When a fragment is exchanged with a non-homologous chromosome. Human Disorders Due to Chromosomal Alterations • Down Syndrome: Trisomy 21, caused by extra chromosome 21 causing 47 chromosomes.

• Symptoms include facial distortion, shorter stature, development issues, increased chance of leukemia and Alzheimers - almost all males are sterile, half of females sterile.

• Klinefelter Syndrome: Caused by XXY, have male sex organs but small testes and sterile. • Symptoms also include breast enlargement, and some female characteristics. • Other forms of trisomy have different affects (XYY, YYY). Disorders Caused by Structurally Altered Chromosomes • Cri du Chat: Deletion of Chromosome 5 • Symptoms include severely intellectually disabled, small head, deformed face, and a mew cry

like a cat. • Chronic Myelogenous Leukemia: Caused by Philadelphia Gene, a gene dramatically shortened

in translocation.