Complex Inheritance and Human Heredity

1. Complex Inheritanceand Human Heredity 11

2. The Big Idea • Human inheritance does not always follow Mendel’s laws.

3. Main Idea #1 • The inheritance of a trait over several generations can be shown in a pedigree.

4. Recessive Genetic Disorders • Remember - recessive traits can be masked by a dominant trait. • Therefore, a person who is heterozygous for a characteristic may be considered a carrier for that trait since it will not be expressed. • Even though a carrier is not affected by the trait, they can pass the trait on to future generations. • In order for the recessive trait to show in the individual, the person must be homozygous recessive.

5. Common Recessive Disorders • Cystic Fibrosis • Affects the mucus-producing glands, digestive enzymes, and sweat glands • Chloride ions are not absorbed into the cells of a person with cystic fibrosis but are excreted in the sweat • Without sufficient chloride ions in the cells, a thick mucus is secreted • The mucus clogs ducts in the pancreas, causes problems with digestion, and blocks some of the tiny passages in the respiratory system.

6. Common Recessive Disorders • Albinism • Caused by altered genes, resulting in the absence of melanin (color pigment) in the hair and eyes • Common characteristics include: white hair, very pale skin, and pink pupils • Tay-Sachs Disease • Caused by the absence of an enzyme responsible for breaking down fatty acids • The fats begin accumulating in the brain, destroying brain cells and mental deterioration.

7. Dominant Genetic Disorders • Some genetic disorders are controlled by a dominant allele. • People who have only one copy of this allele will display the trait. Only those who are homozygous recessive will not have the disorder. • Huntington’s Disease • Disease that gradually destroys the nervous system. Symptoms usually begin between the ages of 30 and 50. • Achondroplasia is a genetic condition that causes small body size and limbs that are comparatively short. This is the most common form of dwarfism.

8. Pedigrees • A pedigree is a diagram that traces the inheritance of a particular trait through several generations. • Males are represented by squares, and females by circles. • One who is affected by the trait will have a symbol that is colored in, while those that are unaffected by the trait will have an unfilled symbol.

9. Main Idea #2 • Complex inheritance of traits does not follow inheritance patterns described by Mendel.

10. Complex Patterns of Inheritance • Incomplete Dominance • The heterozygous phenotype is an intermediate phenotype between the two homozygous phenotypes.

11. Complex Patterns of Inheritance • Codominance • Both alleles are expressed in the heterozygous condition.

12. Complex Patterns of Inheritance • Another example of codominance is sickle-cell disease. • Changes in hemoglobin cause blood cells to change to a sickle shape. • People who are heterozygous for the trait have both normal and sickle-shaped cells.

13. Complex Patterns of Inheritance • Multiple Alleles • Some traits are controlled by more than 2 alleles for a particular gene. • Human blood groups are controlled by 3 alleles - IA, IB, and i. IA and IB are codominant to each other, and dominant to i. • There are 4 possible blood types - A (IAIA or IAi), B (IBIB or IBi), AB (IAIB), and O (ii).

14. Complex Patterns of Inheritance • Sex Determination • Sex chromosomes determine an individual’s gender. • The X chromosome is much larger than the Y chromosome. • Therefore, it makes sense to assume that X chromosomes carry more genes and genetic information than the Y chromosomes.

15. Sex-Linkage • The presence of a gene on a sex chromosome is called sex linkage. • Genes found on the X chromosome are called X-linked, and genes found on the Y chromosome are called Y-linked.

16. Example of Sex-Linked Trait • Colorblindness is an X-linked trait, carried by sex chromosomes. • The allele for colorblindness is represented by XC, while the normal allele is X. • A female must have two XC alleles in order to be colorblind, but if a male carries the XC allele, he will definitely be colorblind.

17. Colorblindness Test • In each of the circles, see if you can identify a hidden number. A person with normal vision will see the numbers, while a person who is colorblind will see only the circle.

18. Another Example of Sex-Linkage • Hemophilia is another example of a sex-linked trait. • This rare disease causes blood to not clot properly. • Failure of blood to clot may result in death. • This trait is located on the X chromosome. • Males are more likely to display this disorder since they only have to have one copy of the allele.

20. Polygenic Traits • Polygenic traits arise from the interaction of multiple pairs of genes. • Examples include skin color, height, and body mass.

21. Environmental Factors • Sometimes, the environment affects how a trait is expressed. • These influences include: • environmental factors • diet and exercise • sunlight and water • temperature

22. Twin Studies • Twin studies can help scientists determine if a characteristic is caused by genetics or by environmental conditions (Nature vs. Nurture). • Traits that appear frequently in identical twins are at least partially controlled by heredity. • Traits expressed differently in identical twins are strongly influenced by environment.

23. Main Idea #3 • Chromosomes can be studied using karyotypes.

24. Karyotypes • A karyotype is a picture of a person’s chromosomes. • Chromosomes are taken from the stage of metaphase and stained. • They are then placed in order of decreasing size. The last pair is the sex chromosome pair. • Disorders caused by an incorrect number of chromosomes or malformed chromosomes are the easiest to identify using a karyotype.

25. Normal Karyotypes

26. Telomeres • Telomere caps consist of DNA associated with proteins. • They serve a protective function for the structure of the chromosome.

27. Nondisjunction • Homologous chromosomes separate during the final steps of meiosis to ensure that each developing sperm or egg receives one copy of each chromosome. • Nondisjunction is the failure of a pair of homologous chromosomes to separate properly. • Resulting sex cells will either have an extra chromosome (trisomy) or a missing chromosome (monosomy).

28. Down Syndrome • One of the most common disorders due to nondisjunction is Down syndrome, also known as trisomy 21. • Down syndrome occurs when the person has a third copy of the 21st chromosome. • Common features include: short stature, heart defects, distinctive facial features, and mental disability.

29. Disorders Due to Nondisjunction of Sex Chromosomes Klinefelter Syndrome (XXY) Jacob’s Syndrome (XYY) Trisomy X Syndrome (XXX) Turner’s Syndrome (XO)

31. Types of Mutations • Mutations can involve an entire chromosome or a single DNA nucleotide, and they may take place in any cell. • Germ-cell mutations occur in an organism’s gametes (germ cells). • These do not affect that organism, but may be passed on to their offspring the germ cell becomes fertilized. • Somatic mutations occur in an organism’s body cells and can affect the organism. • These mutations cannot be passed on to offspring. • Some examples include skin cancer and leukemia. • Lethal mutations cause death, usually before birth.

32. Chromosome Mutations • Deletion (1)- results in the loss of a piece of chromosome due to the breakage of that chromosome; genetic information will be lost • Duplication (2)- results in the copying of a segment of the chromosome • Inversion (3)- a segment of a chromosome breaks off and reattaches itself to the chromosome in a reversed order

33. Chromosome Mutations • Insertion (1) - a segment of a chromosome breaks off and reattaches itself to another homologous chromosome • Translocation (2) - segments of chromosomes break off and exchange places on different chromosomes

34. Chromosome Mutations • Nondisjunction is also a type of chromosome mutation. It affects the total number of chromosomes instead of pieces of individual chromosomes.