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Heredity PowerPoint PPT Presentation

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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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  • 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


  • 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

Human genetics

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


  • 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



  • 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?













  • The possible blood types for the offspring would be AB, A, B or O

  • Additionally, the Rh+ factor is a dominant allele

Pedigree charts

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

Human traits

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.


  • 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

Genetic disorders

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)

Cystic fibrosis

Cystic Fibrosis

  • Caused by a recessive allele

  • Sufferers of cystic fibrosis produce a thick, heavy mucus that clogs their lungs and breathing passageways


  • 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.

From mutation to disease

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


  • Many other conditions are caused by recessive alleles


  • 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


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

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