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Chapter 14: Human Inheritance

Explore the complexities of studying human inheritance, including the role of chromosomes, the transmission of traits, and the use of pedigrees to understand family history.

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Chapter 14: Human Inheritance

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  1. Chapter 14: Human Inheritance Section 14-1: Human Chromosomes

  2. Studying Humans? • Human heredity is difficult to study because they produce few offspring, have a long life span, and cannot be grown in a lab

  3. Karyotypes • Need to study the human genome – the full set of genetic information an organism carries in its DNA • Genomic study starts with chromosomes • To see chromosomes clearly, scientists, take a picture of the cell going through mitosis, cut the chromosomes out the picture and make a karyotype • A karyotype is a diagram that shows the complete set of diploid chromosomes grouped together in pairs and arranged in order of decreasing size • Normal human karyotype contains 46 chromosomes (23 pairs)

  4. Sex Chromosomes • Two of the 46 (one pair) are sex chromosomes (determines an individual’s sex) • Females have two X chromosomes • Males have one X and one Y • Roughly a 1:1 ratio of females to males

  5. Sex Chromosomes • The X chromosome is large and carries more than 1200 genes, most of which are necessary for both sexes • The Y chromosome is much smaller, only carrying 140 genes, controlling male sex determination and sperm development

  6. Autosomal Chromosomes • The other 44 human chromosomes are known as autosomal chromosomes, or autosomes • Biologists write 46,XX for females and 46,XY for males

  7. Transmission of Human Traits • Many human traits are Mendelian (simple dominance) • Ex. - gene known as MC1R helps determine skin and hair color • Some of MC1R’s recessive alleles produce red hair, while dominant alleles for the MC1R gene help produce darker hair colors • Ex. – Rhesus factor, or Rh blood group • The allele for Rh factor comes in two forms: Rh+ and Rh-. • Rh+ is dominant, so an individual with both alleles (Rh+/Rh-) has Rh positive blood. • Rh negative blood is found in individuals with two recessive alleles (Rh+/Rh-)

  8. Codominant and Multiple Alleles • ABO blood group, determined by 3 alleles IA, IB, and i • Alleles IA and IBare codominant - produce molecules known as antigens on the surface of red blood cells. • Individuals with alleles IAand IBproduce both A and B antigens, making them blood type AB

  9. Codominant and Multiple Alleles • The i allele is recessive – no antigens • If a patient has AB-negative blood, it means the individual has IAand IBalleles from the ABO gene and two Rh- alleles from the Rh gene

  10. Sex-Linked Inheritance • Genes located on X or Y chromosomes are called sex-linked genes • Genes on Y chromosome are found only in males, passed directly from father to son • Genes on X chromosome are found in both sexes • Males only get one X

  11. Sex-Linked Inheritance • There are 3 genes responsible for color vision, all located on the X chromosome • In males, any of these genes being recessive causes colorblindness, the most common form affecting 1 in 12 males • This condition only occurs in 1 in every 200 females because she must have two recessive alleles • The recessive phenotype of a sex-linked genetic disorder tends to be much more common among males than among females (hemophilia, DMD) • Males cannot be heterozygous for genes carried on the X-chromosome!!

  12. X-Chromosome Inactivation • Just one X chromosome is enough for cells in males, but females have 2 • In female cells, most of the genes in one of the X chromosomes are randomly switched off, forming a dense region in the nucleus known as a Barr body • Barr bodies are not found in males

  13. X-Chromosome Inactivation • Happens in other mammals as well – calico cats • Gene that controls the color of coat spots is located on the X chromosome • In a females, one X chromosome may have an allele for orange spots and the other X chromosome may have an allele for black spots • Depending on which X chromosome is turned off in which cell, the spots can be either color • As a result, the cat’s fur has a mixture of orange and black spots

  14. X-Chromosome Inactivation • Male cats only have one X chromosome, so they can only have spots of one color • If a cat’s fur has 3 colors, it must be a female!

  15. Human Pedigrees • A pedigree is a diagram that shows the inheritance of a single trait through multiple generations of a family • It shows the relationship betweens parents, siblings, and offspring • Based on family history and medical records

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