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Chapter 12 Inheritance Patterns and Human Genetics

Chromosomes. Early 1900's Thomas Hunt Morgan worked with fruit flies (Drosophila melanogaster)Observed that in males and females, some chromosomes were identical to each other and one pair was different ? X and Y ? sex chromosomes . Sex Determination in Mammals . Sperm cells have an equal chance of containing an X as a Y chromosomes (males are XY)Egg contains only an X chromosome (females are XX)50% chance of male offspring, 50% chance of female offspring.

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Chapter 12 Inheritance Patterns and Human Genetics

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    1. Chapter 12 – Inheritance Patterns and Human Genetics L1 Biology, 2010-2011

    2. Chromosomes Early 1900’s Thomas Hunt Morgan worked with fruit flies (Drosophila melanogaster) Observed that in males and females, some chromosomes were identical to each other and one pair was different – X and Y – sex chromosomes

    3. Sex Determination in Mammals Sperm cells have an equal chance of containing an X as a Y chromosomes (males are XY) Egg contains only an X chromosome (females are XX) 50% chance of male offspring, 50% chance of female offspring

    4. Sex Determination in Mammals In male mammals, the Y chromosome contains a gene called SRY (Sex-determining Region Y) that codes for a protein that causes gonads (organ that makes gametes) of an embryo to develop as testes Female embryos do not have SRY, so the gonads develop as ovaries

    5. Sex-Linked Traits A trait that is coded for by an allele on a sex chromosome X chromosome is larger than Y, so there are more X-linked traits than Y-linked Most X-linked traits have no counterpart on the Y chromosome, so a male who carries a recessive allele on the X chromosome will exhibit the sex-linked trait

    7. Sex-Linked Traits In humans… Male inherits his X chromosome from his mother Colorblindness is a recessive X-linked disorder in which an individual cannot distinguish certain colors, such as red and green

    8. If mom’s a carrier: Her sons have a 50% chance of having colorblindness Her daughters have a 50% chance of being carriers for colorblindness

    9. Please do “Exercise 12”

    10. Linked Genes Law of Independent Assortment (Mendel) assumes that alleles are on different chromosomes There are more than 46 genes, so some genes will be found on the same chromosomes and therefore could be inherited together

    11. Linked Genes - Morgan Two fly genes – body color and wing length G = gray (dominant) g = black L = long wings (dominant) l = short wings Crossed gray-bodied long-winged flies (GGLL) with black-bodied short-winged flies (ggll) We know that GL and gl are each on the same chromosome All F1 were gray-bodied with long wings (GgLl)

    12. Linked Genes - Morgan Morgan crossed F1s – GgLl x GgLl Possible gametes: GL, gl (for each parent) F2s occurred in a phenotypic ratio of three gray long-winged flies to one black short-winged fly If the alleles had assorted independently, a 9:3:3:1 ration would have been observed

    13. Linked Genes Crossing-over – exchange of pieces of the DAN between homologous chromosomes; rearranges allele combinations

    14. Polygenic Some genes have two or more alleles that determine the trait, such as blood type or attached earlobes Most human characteristics are polygenic – influenced by several genes Ex: skin color – 3 to 6 genes – each gene controls the amount of melanin in the skin (brownish-black pigment in skin); each of the genes has two alleles, one that produces low amounts of melanin and the other that produces high amounts; the final amount of melanin in a person’s skin comes from the combination of these alleles Ex: eye color, hair color, height

    15. Complex Characteristics Many human traits are complex characteristics – characters that are influenced strongly both by the environment and by genes Ex: height – polygenic but also influenced by factors such as nutrition and disease Ex: skin color – being in the sun normally makes a person’s skin darker

    16. Multiple Alleles Many genes have more than two alleles Ex: blood type in humans – alleles: IA, IB, i IA and IB also are codominant IA and IB code for sugars on the surface of red blood cells where i does not

    17. Please do “Exercise 10”

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