Mendelian Inheritance of Human traits

1. Mendelian Inheritance of Human traits • A pedigree is a graphic representation of genetic inheritance. • Looks very similar to a family tree. • If III-2 marries a person with the same genotype as individual I-1, what is the chance that one of their children will be afflicted with hemophilia? • 0% I 1 2 • What type of inheritance pattern does the trait represented by the shaded symbols illustrate? • Sex linked II 2 1 3 4 Homozygous dominant Homozygous recessive • What is the relationship between I-1 and III-2? • Grandmother-grandson • What would these be classified as? • Carriers III

2. Pedigrees cont. I II III • What is the probable mode of inheritance? • Sex linkage • What do you know about III-1’s mother? • She was a carrier IV • How many offspring in the III generation show the normal trait? • 4

3. A child is diagnosed with a rare genetic disease. Neither parent has the disease. How might the child have inherited the disorder? • The disorder is recessive and carried by both parents • Most genetic disorders are caused by recessive alleles. • Cystic fibrosis and Tay-Sachs disease are typical of recessive disorders concentrated in ethnic groups. • PKU is the failure to metabolize phenylalanine. • A homozygous PKU newborn appears healthy at first because its mother’s normal enzyme level prevented phenylalanine accumulation during development. • Following the detection of PKU, dietary adjustments are made in order to prevent mental retardation.

4. Simple Dominant Heredity • In Mendelian inheritance, a single dominant allele is inherited from one parent is all that is needed for a person to show the dominant trait. • Phenotypic traits that result in single dominant allele: • Polydactyly • Tongue rolling • Hitchhiker’s thumb • Thick lips • Hair in the middle section of the fingers • Huntington’s disease • Huntington’s disease is the progressive deterioration of the nervous system. • Every child of an affected individual has a 50% chance of being affected and then a 50% chance of passing the defective allele to his or her own child.

5. When Heredity Follows Different Rules • Incomplete dominance is when the phenotype of the heterozygous is intermediate between those of the two homozygous. • For example, if a homozygous red-flowered snapdragon plant is crossed with a homozygous white-flowered snapdragon plant, all of the offspring will have pink flowers. (Figure 12.7 pg 322) • A cross a white rooster and a black hen results in 100% blue Andalusian offspring. When 2 of these blue offspring are mated the probable phenotypic ratio seen in their offspring would be? • WW(white) X ww(black)=blue (all would be Ww) • Then you would do a cross between those offspring: w W 25% would be white 50% would be blue 25% black WW Ww W Ww ww w

6. Codominant alleles cause the phenotypes of both homozygotes to be produced in heterozygous individuals • Both alleles are equally expressed • Examples: • Figure 12.8 all of the offspring would be checkered; some feathers are black and other feathers are white • Roan cattle-coat consists of red and white hairs • Traits controlled by more than two alleles have multiple alleles. • A trait controlled by 4 alleles

7. There are 22 pairs of matching homologous chromosomes call autosomes. • The 23rd pair of chromosomes that differ in males and females are called sex chromosomes. • *A male is said to be hemizygous for genes on the X chromosome. Explain why you think this term was chosen. • The prefix hemi- means half. Because only one of the male’s 2 sex chromosomes is an X chromosome, only half of his sex chromosomes can carry the genes.

8. Traits controlled by genes located on sex chromosomes (X or Y chromosomes) are called sex-linked traits. • Fig 12.11 pg 325 • Polygenic inheritance is the inheritance pattern of a trait that is controlled by two or more genes. • More than two phenotypes result from both multiple and polygenic inheritance. • Human traits skin color and height would show similar inheritance pattern and frequency of distribution. Fig 12.12 pg 326 • *How does polygenic inheritance differ from Mendelian inheritance? • In Mendelian inheritance, traits are determined by dominant and recessive paired alleles of single genes. In polygenic inheritance, a trait is controlled by two or more genes.

9. *Discuss how the external environment of an organism can affect gene function. • Pg 327 • Temperature, nutrition, light, chemicals, and infectious agents all can influence gene expression. • *Discuss how the internal environment of an organism can affect gene function/ • Pg 327 • Age • Gender • Sex hormones

10. Complex Inheritance of Human Traits • Individuals afflicted with sickle-cell anemia suffer tissue damage resulting from oxygen deprivation. • The blood types A, B, AB, and O are the results of multiple allelic inheritance. • A man heterozygous for blood type A marries a woman heterozygous for blood type B. The chance that their first child will have type O blood is ? I A i IA IB=AB IB i=B IA i=A ii=O Answer: 25% IA IB IB i I B IA i ii i

11. Which baby belongs to each couple? • Mrs. Page—B Mr. Page—AB • Mrs. Baker—B Mr. Baker—A • Baby #1—A • Baby #2—O • Page’s • Baker’s IA IB=Blood type AB IB IB=Blood type B IA i=Blood type A IB i=Blood type B IA IB • Page’s • Only baby 1 could be theirs. IB IA IB IB IB IA i IB i i IA IB=Blood type AB IA i=Blood type A IB i=Blood type B ii=Blood type O IB i • Bakers • Either baby could be theirs IA IA IB IA i i IB i ii

12. Examples of sex-linked traits: • Hemophilia(Royal hemophilia) • Red-Green color blindness • Red-Green color blindness is caused by a recessive allele. • Because the gene for red-green color blindness is located on the X-chromosome, it is normally not possible for a color blind father to pass the gene to his son. (Fig 12.17 pg 332) • A karyotype can be valuable in pinpointing cases of aneuploidy. • Aneuploidy—having lower or higher than the normal diploid number of chromosomes. • Down Syndrome and Turner Syndrome can be detected by karyotyping