Chromosomes and Inheritance

1. Chromosomes and Inheritance 12.1

2. review:Chromosomes: • Thomas Sutton in 1902 proposed that genes are located on chromosomes • Called the Chromosome Theory of Inheritance • For most of the life of the cell, chromosomes are too elongated to be seen under a microscope & are  called chromatin • Before a cell gets ready to divide, each chromosome is duplicated & condenses into short structures • Each chromosome is composed of a single, tightly coiled DNA molecule  • The two DNA strands are homologous (duplicates) and are held together by the centromere • While they are still attached, the duplicated chromosomes are called sister chromatids

4. Fertilization restores the diploid chromosome number and paired condition for alleles in the zygote • Chromosomes can be categorized as two types --- autosomes & sex chromosomes • Autosomes are non-sex chromosomes that are the same number and kind between sexes

5. Sex chromosomes determine if the individual is male or female • Sex chromosomes in the human female are XX and those of the male are XY • Males produce X- and Y-containing gametes; so males determine the sex of offspring 

6. Chromosome Numbers: • All animals have a characteristic number of chromosomes in their somatic or body cells called the diploid (or 2n) number. • The gametes or sex cells (egg & sperm) contain half the number of chromosomes as a body cell; known as the haploid number (n) of chromosomes

8. Sex Linkage: • Thomas Hunt Morgan worked with fruit flies & confirmed that  genes were on chromosomesa. Fruit flies are cheaply raised in common laboratory glasswareb. Females only mate once and lay hundreds of eggsc. Fruit fly generation time is short, allowing rapid experiments • Experiments involved fruit flies with XY system similar to human system

9. Sex-linked cont’ • Besides genes that determine sex, sex chromosomes carry many genes for traits unrelated to sex • X-linked gene is any gene located on the X chromosome that are missing on the Y chromosome • X-linked alleles are designated as superscripts to X chromosome • Newly discovered mutant male fruit fly had white eyes

10. Mutant White-eyed  & Wild, Red-eyed

11. Fruit fly experiment cont’ • Cross of white-eyed male with dominant red-eyed female yield expected 3:1 red-to-white ratio; however, all white-eyed flies were males • An allele for eye color on the X but not Y chromosome supports the results of the cross • Heterozygous females are carriers that do not show the trait but can pass it on • Males are never carriers but express the one allele on the X chromosome • Red-green color-blindness is X-linked recessive

12. Sex-linked problem • What are the results of crossing a colorblind male with a female carrier for colorblindness? XCXc       x    Xc Y female male

13. Punnett square

14. Results: • Genotypes:   XCXC ,XCY, XCXc, XcY • Genotypic Ratio: 1:1:1:1 • Phenotypes:normal vision female, normal vision male, colorblind female, colorblind male

15. Linked genes: • Each chromosome has 1000's of genes • All genes on a chromosome form a linkage group that stays together except during crossing-over • Some genes located on the same chromosome tend to be inherited together  • Linked genes were discovered by Thomas Hunt Morgan while studying fruit flies • Linked alleles do not obey Mendel's laws because they tend to go into the gametes together • Crosses involving linked genes do not give same results as unlinked genes

16. Chromosome Mapping: • Recombinants result from chromosome crossing over during prophase I of meiosis • Geneticists can use recombination data to map a chromosome's genetic loci (position on a chromosome) • A genetic map lists a sequence of genetic loci along a particular chromosome • Alfred Sturtevant, a student of Morgan, reasoned that different recombination frequencies reflect different distances between genes on a chromosome • The farther apart genes are, the greater likelihood of crossing-over • The closer together two genes are, the less likely of crossing-over occurring • A map unit equals 1% recombination frequency • If 1% of crossing-over equals one map unit, then 6% recombinants reveal 6 map units between genes • To determine the frequency of recombinants, the following formula is used:

17. Number of recombinants x 100% Recombination Frequency=  ---------------------------------------------      Total Number of Offspring

18. Humans have few offspring and a long generation time so biochemical methods are used to map human chromosomes (Human Genome Project)

19. Chromosome Mutations: • Mutations change in the DNA of an organism • They increase the number of variations that occur • Germ-cell mutations – changes in gametes don’t affect organism but may be passed onto offspring • Somatic mutations – occur in body cells and affect the organism • E.g. cancer • Lethal mutations – cause death before birth • Chromosomal mutations include changes in chromosome number and/or structure

20. Chromo mutations cont’ • Deletions occur when the end of a chromosome breaks off & is lost • E.g. Cri du chat syndrome– due to a deletion of a portion of chromosome 5 • results in retardation & a cat-like cry

21. Chromo mutations cont’ • Inversion occurs when a piece of a chromosome breaks off & reattaches to the same place but in the reverse order • Translocation occurs when a chromosome segment breaks off & attaches to a different chromosome • Nondisjunction – failure of chromosome to separate from its homologue during meiosis

22. Gene Mutations: • Change in genes caused by change in structure of the DNA • DNA bases may be substituted, added, or removed to cause gene mutation

23. point mutation – substituting, adding or removing genes

24. Sickle cell anemia – (pt mutation- substitution) red blood cells are C-shaped so can't carry as much oxygen - example of a gene mutation in African Americans - 1:500

25. frame shift mutation – when genes are added or removed

26. Human Genetics 12.2

27. Pedigrees: • Also called a family tree • Squares represent males and circles represent females • Horizontal lines connecting a male and female represent mating • Vertical lines extending downward from a couple represent their children • A shaded symbol means the individual possess the trait • Half-shaded symbols are carriers

28. a bit of history • In humans, another well-known X-linked traits is hemophilia (free bleeders that lack clotting factors in their blood) • One of the most famous genetic cases involving hemophilia goes back to Queen Victoria who was a carrier for the disorder and married Prince Albert who was normal • Their children married other royalty, and spread the gene throughout the royal families of Europe

30. Genetic disorders - diseases or debilitating conditions with a genetic basis • Huntington’s disease – caused by dominant allele on autosome • Symptoms – forgetfulness and irritability in 30’s • Loss of muscle control, severe mental illness, death

31. Patterns of Human Traits Single Allele Dominant Huntington’s disease Acondroplasia (dwarfism) Cataracts Polydactyly (extra fingers/toes) Single Allele Recessive Albinism Systic fibrosis Phenylketonuria (PKU) Hereditary deafness X-Linked Colorblindness Hemophilia Muscular dystrophy Icthyosis simplex (scaly skin) Polygenic Skin, hair, eye color Foot size Nose length Height Multiple Alleles ABO blood groups • Single-allele traits – controlled by a single allele of a gene • Multiple-allele traits – controlled by 3+ alleles that code for a single trait • Polygenic – controlled by 2+ genes

32. Nondisjunction • Monosomy occurs when an individual has only one of a particular type of chromosome • (45 chromosomes instead of 46) • E.g. Turner syndrome (X0) • Trisomy occurs when an individual has three of a particular type of chromosome • (47 chromosomes instead of 46) • E.g. Klinefelter's Syndrome (XXY) • Down Syndrome or Trisomy 21 (individual has three 21st chromosomes)

33. Duplications – occur when a section of a chromosome is doubled • Fragile X Syndrome caused by an abnormal number of repeats (CCG) results in retardation & long, narrow face becomes more pronounced with age

34. Environmental factors including radiation, chemicals, and viruses, can cause chromosomes to break causing a change in chromosomal structure

35. Tay-Sachs – a disorder where the nervous system deteriorates • a fatal gene mutation in Jewish people of Central European Descent • Phenylketonuria or PKU – gene is unable to synthesize a single enzyme necessary for the normal metabolism of phenylalanine • results in death