Chromosomes, Mapping, and the Meiosis-Inheritance Connection

1. Chromosomes, Mapping, and the Meiosis-Inheritance Connection Chapter 12

2. 2 Chromosome Theory Chromosomal theory of inheritance developed in 1902 by Walter Sutton proposed that genes are present on chromosomes based on observations that homologous chromosomes pair with each other during meiosis supporting evidence was provided by work with fruit flies

3. 3 Chromosome Theory T.H. Morgan isolated a mutant white-eyed Drosophila red-eyed female X white-eyed male gave a F1 generation of all red eyes Morgan concluded that red eyes are dominant

4. 4 Chromosome Theory Morgan crossed F1 females X F1 males F2 generation contained red and white- eyed flies but all white-eyed flies were male testcross of a F1 female with a white-eyed male showed the viability of white-eyed females Morgan concluded that the eye color gene is linked to the X chromosome

5. Chromosomal basis of sex linkage White-eyed male flies X red-eyed females F1 flies all have red eyes F2 flies, all of the white-eyed flies are males because the Y chromosome lacks the white gene 5

6. 6 Sex Chromosomes Sex determination in Drosophila is based on the number of X chromosomes 2 X chromosomes = female 1 X and 1 Y chromosome = male Sex determination in humans is based on the presence of a Y chromosome 2 X chromosomes = female having a Y chromosome (XY) = male

7. 7 Sex Chromosomes In many organisms, the Y chromosome is greatly reduced or inactive. genes on the X chromosome are present in only 1 copy in males sex-linked traits: controlled by genes present on the X chromosome Human X-linked disorders Color blindness, Muscular dystrophy, Hemophilia, Fragile X syndrome Sex-linked traits show inheritance patterns different than those of genes on autosomes.

8. Royal Hemophilia Pedigree 8

9. 9 Sex Chromosomes Dosage compensation ensures an equal expression of genes from the sex chromosomes even though females have 2 X chromosomes and males have only 1. In each female cell, 1 X chromosome is inactivated and is highly condensed into a Barr body. Females heterozygous for genes on the X chromosome are genetic mosaics.

10. Genetic basis behind a calico cat 10

11. 11 Chromosome Theory Exceptions Mitochondria and chloroplasts contain genes. traits controlled by these genes do not follow the chromosomal theory of inheritance genes from mitochondria and chloroplasts are often passed to the offspring by only one parent

12. 12 Chromosome Theory Exceptions Maternal inheritance: uniparental (one-parent) inheritance from the mother the mitochondria in a zygote are from the egg cell; no mitochondria come from the sperm during fertilization in plants, the chloroplasts are often inherited from the mother, although this is species dependent

13. Human X Chromosome Gene Map 13

14. 14 Human Genetic Disorders Some human genetic disorders are caused by altered proteins. the altered protein is encoded by a mutated DNA sequence the altered protein does not function correctly, causing a change to the phenotype the protein can be altered at only a single amino acid (e.g. sickle cell anemia)

15. Sickle-Cell Anemia 15

16. 16 Human Genetic Disorders Some genetic disorders are caused by a change in the number of chromosomes. nondisjunction during meiosis can create gametes having one too many or one too few chromosomes fertilization of these gametes creates trisomic or monosomic individuals Down syndrome is trisomy of chromosome 21

17. Down Syndrome 17

18. 18 Human Genetic Disorders Nondisjunction of sex chromosomes can result in:

19. Abnormalities in the # of sex chromosomes 19

20. 20 Human Genetic Disorders genomic imprinting occurs when the phenotype exhibited by a particular allele depends on which parent contributed the allele to the offspring a specific partial deletion of chromosome 15 results in: Prader-Willi syndrome if the chromosome is from the father Angelman syndrome if it’s from the mother

21. 21 Human Genetic Disorders Genetic counseling can use pedigree analysis to determine the probability of genetic disorders in the offspring. Some genetic disorders can be diagnosed during pregnancy. amniocentesis collects fetal cells from the amniotic fluid for examination chorionic villi sampling collects cells from the placenta for examination

22. Amniocentesis 22

23. Chorionic villi sampling 23