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Heredity

Heredity. Chromosomes. We discussed earlier in this unit, the usage of karyotype charts Humans have 23 pairs of chromosomes 1 sex chromosome pair 22 autosome pairs Recall that males have one X and one Y, while females have two X’s

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Heredity

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  1. Heredity

  2. Chromosomes • We discussed earlier in this unit, the usage of karyotype charts • Humans have 23 pairs of chromosomes • 1 sex chromosome pair • 22 autosome pairs • Recall that males have one X and one Y, while females have two X’s • These split during meiosis, and we can use a Punnett square to determine the possible outcomes for sex of offspring

  3. From this, we can see that the mother can only supply an X chromosome • The sex of the child is solely due to the father • Sperm cells should be produced in equal numbers for X and Y

  4. Human Genetics • Recall human blood type has multiple alleles • The traditional convention for expressing dominance and recessiveness no longer works • Alleles are often expressed as superscripts • Both A and B types are codominant, and O is recessive • A is IA, B is IB, and O is i • The following chart summarizes the genotypes and phenotypes

  5. Because A and B are codominant, they both show up, but do not blend • The individual will be AB • A lack of either results in O

  6. Example • A male heterozygous for blood type A plans to have children with a female who is heterozygous for B. What possible blood types could their offspring have?

  7. IBi IB i IAIB IAi IA IAi i IBi ii

  8. The possible blood types for the offspring would be AB, A, B or O • Additionally, the Rh+ factor is a dominant allele

  9. Pedigree Charts • After identifying the nature of a trait, geneticists often look at family history • By understanding the phenotypes of certain members of a family, they can gather more info about others • This is organized in a pedigree chart

  10. Human Traits A square represents a male. A circle represents a female. A vertical line and a bracket connect the parents to their children. A horizontal line connecting a male and a female represents a marriage. A shaded circle or square indicates that a person expresses the trait. A circle or square that is not shaded indicates that a person does not express the trait.

  11. These only work for traits that are thought to be controlled by genetics alone • Also works best on traits the are due to one gene

  12. Genetic Disorders • Many conditions are due to recessive alleles • These will only manifest themselves if a dominant allele is present • An example is cystic fibrosis (CF)

  13. Cystic Fibrosis • Caused by a recessive allele • Sufferers of cystic fibrosis produce a thick, heavy mucus that clogs their lungs and breathing passageways

  14. The most common allele that causes cystic fibrosis is missing 3 DNA bases. • As a result, the amino acid phenylalanine is missing from the CFTR protein.

  15. From Mutation to Disease • Normal CFTR is a chloride ion channel in cell membranes • Abnormal CFTR cannot be transported to the cell membrane • If it does, it will not transport Cl- as easily • Part 1 • Part 2

  16. Many other conditions are caused by recessive alleles

  17. Other conditions arise from codominant alleles • In these cases, the heterozygotes have a different phenotype • You saw this with thalassemia in question 12 • Sickle cell disease is another example • Individuals that are heterozygous for this usually have normal blood cells, but are resistant to malaria

  18. Finally, some conditions are caused by dominant alleles, although it is uncommon

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