Chapter 12 Chromosomes & Human Inheritance

1. Chapter 12Chromosomes & Human Inheritance OCC BIO-114

2. In this chapter… • you will learn how biologists use their knowledge of DNA & chromosome behavior to study how traits are inherited & expressed.

3. Sex-determining Chromosomes and Linkage • sex chromosomes • autosomes

4. Sex Chromosomes • It’s all about the “X” and the “Y”

6. Sex determination and Inheritance of Sex-linked genes: • Mammals/ many insects= male/female same number of chromosomes-one pair= sex chromosomes • different appearance and genetic composition • XX= female • XY= male • Humans: • Autosomes = # 1-22 • Sex chromosome = #23 • sex chromosome carried by sperm determines sex of offspring ( X or Y sperm)

7. Sex-Linkage • In 1910, Thomas Hunt Morgan - Studied inheritance patterns of fruit fly, Drosophila melanogaster, discovered presence of a white eye in certain individuals. • Since this was a distinctive feature, Morgan decided to study the inheritance pattern for this recessive eye color.

8. Sex-Linkage • Morgan made several crosses using a white-eyed male, expecting the standard Mendelian results. He did not get them. • While the ratio of 3:1 was obtained, all of the white-eyed second generation offspring were male flies. • All females had red eyes (and 25% of the males also had red eyes).

9. Sex-Linkage • Morgan did a series of reciprocal crosses of white-eye males with red-eye females and red-eye males with white-eye females. • He concluded that the gene for eye color was located on the X chromosome. • Males passed the trait their daughters (on their solitary X chromosome) & mothers passed the trait sons. • White eyed females could also pass the white eye allele to their daughters, but if the father fly had red eyes, the eye color of the daughters would be red, while the eye color of the sons of white-eyed females would always be white.

11. Sex-Linkage • Morgan concluded that eye color was related to sex, & that the sex-determining chromosomes also had genes that were unrelated to gender determination. • Prior to Morgan's discovery, no one knew that genes unrelated to gender were also located on these chromosomes.

12. Sex-Linkage • These other traits are said to be sex-linked because they are inherited along with the sex of the individual. • Because the X and Y chromosome are not exactly matching, the X chromosome can have genes that are not located on the Y chromosome, and vice-versa. • Some of these genes are unrelated to the sexual characteristics, but are inherited with the sex-determination. This is referred to as sex-linkage.

13. sex-linked (X-linked) • genes on one sex chromosome; not on other • Y= carries relatively few genesX= many genes; some not specifically related to female traits Ex: genes for: color vision; • blood clotting; structural proteins • female XX= homozygous or hybrid (dominant/recessive) • male XY= fully expresses all alleles on single X (whether dominant or recessive) Ex: color blindness; hemophilia; muscular dystrophy

14. Some human sex-linked traits are • Hemophilia (X) • Hairy ear rims (Y) • Red-green color blindness (X) (fig.12.9) • Duchene muscular dystrophy (X)

15. X-Linked • Only found on X chromosome • Colorblindness – red/green, most common in males (8%) • Hemophilia – blood clotting, affects males • Duchene Muscular Dystrophy – weakens & destroys muscle tissue *Not all X-Linked traits are diseases = Only a few of hundereds of genes on X chrom. Others code for normal functioning proteins

16. The Effect of Recombination on Gene Linkage and Inheritance • Meiosis results in exchange of bits & pieces of DNA between homologous pairs of chromosomes at the chiasmata during prophase I of meiosis. • This process of recombination results in gametes (or meiotic products) that are not identical; some of the linkage groups have been changed by the crossing-over. • As a result of recombination, new allele combinations are formed, and we have more genetic variation.

17. crossing over • segments of homologous chromosomes are exchanged w/ each other at site called chiasmata during meiosis I • forms new gene combinations on both homologous chromosomes • gene combinations for daughter cell different from parent cell

18. Crossing Over and Recombination

20. Cell Mutations • Germ-cell mutation – gametes, passed on to offspring • Somatic mutations – body cells, affect organism. Not passed on to offspring. i.e. leukemia • Lethal mutations – cause death. Are they beneficial???

21. Chromosome mutations(12.5) • Deletion – piece of chrom breaks off & is lost (end); due to virus, radiation, chemicals, or envir factors. Most are lethal • Ex:ABCDEFG becomes ABCFG, Cri-du-chat syndrome (p.192) • Inversion – breaks off & reattaches in reverse • Ex: ABCDEFG becomes ABGFEDC • Translocation – breaks off & reattaches to different (non-homologous chrom) • Ex: ABCDEFG becomes ABCDLMNOP

22. Changes in Chrom # • Nondisjunction – chrom fail to separate during Anaphase (gamete formation) (p.192) • Aneuploidy – 1 extra or less chrom • Monosomic-1 less, Turner syndrome (XO) • Trisomic-1 more • Polyploidy – 3 or more of each type of chrom; lethal for humans

23. Inheritance of Recessive Alleles • Any alteration of a gene, called a mutation, has the potential to inhibit the formation of a needed enzyme. With diploid organisms, however, a mutation most likely affects just one of the homologues, and the second can still code for the appropriate enzyme with little or no phenotypic effect on the individual. • Gene alterations that affect health are called genetic disorders (Table 12.1, p.196). • Those that are just "abnormal" but do not affect physiological health, are called genetic abnormalities i.e. 6 toes • When the genetic alteration causes a host of symptoms, it may be called a syndrome. • Disease –illness caused by infections, not by inheritance

24. Single Allele Traits • Controlled by a single allele of a gene. • >200 human traits governed by single dominant allele. • Ex: Huntington’s Disease (HD) caused by dominant allele located on an autosome = autosomal dominant pattern of inheritance • Read description of HD on p.196, Table 12.1

25. HD continued • Geneticists discovered a genetic marker for HD allele. • Genetic marker is a short section of DNA known to have a close association w/ a particular gene located nearby. Easy to I.D. the HD allele. • If marker is present = 96% chance of dev HD • Parents can be tested for marker before conceiving a child.

26. Other Single-Allele Traits (Table 12.1) • Homozygous recessive (must have 2 copies of recessive allele): • Cystic Fibrosis • Sickle Cell Anemia *these are recessive alleles located on autosomes = Autosomal Recessive pattern of inhertiance

27. polygenic inheritance • Trait that is controlled by 2 or more genes = many different variations • Ex: Skin Color-influenced by additive effects of 3 to 6 genes. Each gene results in certain amount of melanin (brownish-black pigment) • More melanin=darker • Ex: eye color. Light blue eyes=very little melanin • Human height – polygenic, but influenced by environmental factors, such as disease & nutrition

28. Disorders due to Nondisjunction • Occurs during meiosis causes gametes to lack a chromosome or have an extra one (fig 12-10, p. 231) • A zygote w/ 45 chrom. = monosomy • 47 chrom. = trisomy • Often lethal • Trisomy 21 = Down Syndrome – mild to severe mental retardation…

29. Nondisjunction cont. • Males w/ 1 extra X chrom. = Klinefelter’s syndrome (XXY). Some feminine characteristics, mentally retarded & infertile • Turner’s Syndrome – Have a single x chrom. (XO) = female appearance, but do not mature sexually & remain infertile. What happens if zygote only receives a Y chrom.?

30. Detection (fig.12.21, 12.22) • Genetic Screening = Karyotype (picture of person’s chrom. • Amniocentesis – procedure removes amniotic fluid from fetus & tested (14th-16th week of pregnancy) • Chorionic Villi Sampling – Removes fetal cells from chorion fluid (between mothers uterus & fetus) • Ultrasound – Sound waves to observe fetus

31. Detection Cont. • In U.S. 1 out of 10,000 babies is afflicted w/ phenylketonuria (PKU) – body cannot metabolize amino acid phenylalanine = brain damage. • Genetic Counseling – medical guidance for couples at risk

32. 12-10 Human Genetics • Humans have up to 20 times as many genes as Drosophila, & our 23 pairs of chromosomes & are made up of about 100,000 genes. • Geneticists focus on disease-causing genes b/c of concern for human pop.

33. Pedigree Analysis • Pedigree – a family record that shows how a trait is inherited over several generations (fig 12-19). • Certain phenotypes are usually repeated in predictable patterns from 1 generation to the next = patterns of inheritance. • Carriers – individuals who have 1 copy of a recessive allele, but can pass it along to their offspring.

34. How to read a Pedigree • Human pedigrees • Before we consider human Mendelian inheritance it is convenient to consider the symbols used to draw pedigrees. • Generations are numberered from the top of the pedigree in uppercase Roman numerals, I, II, III etc. Individuals in each generation are numbered from the left in arab numberals as subscripts, III1 , III2, III3 etc.

36. Hemophilia pedigree of the European Royal Families

37. Practice Problems • Glencoe

38. References • http://www.scidiv.bcc.ctc.edu/rkr/Biology101/lectures/pdfs/HumanInheritance101.pdf