The Chromosomal Basis of Inheritance

Transcription

1 The Chromosomal Basis of Inheritance Mitosis and meiosis were first described in the late 800s. The chromosome theory of inheritance states: Mendelian genes have specific loci (positions) on chromosomes. Chromosomes undergo segregation and independent assortment. The behaviour of chromosomes during meiosis can account for Mendel s laws of segregation and independent assortment.

2 Figure 5.2a P Generation Y Y R R Yellow-round seeds (YYRR) y y r r Green-wrinkled seeds (yyrr) Meiosis Gametes R Y Fertilization y r

3 Figure 5.2b All F plants produce yellow-round seeds (YyRr). F Generation R r y R r y Y Y LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. R Y r y Meiosis Metaphase I r Y R y LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. R r r R Anaphase I Y y Y y 2 R Y r y Metaphase II r Y R y 2 Gametes Y Y y y Y Y y y R R r r r r R R / 4 / 4 / 4 / 4 YR yr Yr yr

4 Figure 5.2c LAW OF SEGREGATION LAW OF INDEPENDENT ASSORTMENT F 2 Generation An F F cross-fertilization 3 Fertilization recombines the R and r alleles at random. 9 : 3 : 3 : 3 Fertilization results in the 9:3:3: phenotypic ratio in the F 2 generation.

5 Morgan s Experimental Evidence: Scientific Inquiry The first solid evidence associating a specific gene with a specific chromosome came from Thomas Hunt Morgan, an embryologist. Morgan s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel s heritable factors.

6 Morgan s Choice of Experimental Organism Several characteristics make fruit flies a convenient organism for genetic studies. They produce many offspring. A generation can be bred every two weeks. They have only four pairs of chromosomes.

7 Morgan noted wild type, or normal, phenotypes that were common in the fly populations. Traits alternative to the wild type are called mutant phenotypes.

8 Figure 5.3 Wild Type Red Eyes Mutant Type White Eyes

9 Correlating Behaviour of a Gene s Alleles with Behaviour of a Chromosome Pair In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type). The F generation all had red eyes. The F 2 generation showed the 3: red:white eye ratio, but only males had white eyes. Morgan determined that the white-eyed mutant allele must be located on the X chromosome. Morgan s finding supported the chromosome theory of inheritance.

10 Figure 5.4a EXPERIMENT P Generation F Generation All offspring had red eyes. RESULTS F 2 Generation

11 Figure 5.4b CONCLUSION P Generation X X w + w + X Y w F Generation Eggs w w + w + w + w Sperm F 2 Generation Eggs w + w + w + w + w + w w w w + Sperm

12 The Chromosomal Basis of Sex In humans and some other animals, there is a chromosomal basis of sex determination. In humans and other mammals, there are two varieties of sex chromosomes: a larger X chromosome and a smaller Y chromosome. Only the ends of the Y chromosome have regions that are homologous with corresponding regions of the X chromosome. The SRY gene on the Y chromosome codes for a protein that directs the development of male anatomical features.

13 Figure 5.5 X Y

14 Females are XX, and males are XY. Each ovum contains an X chromosome, while a sperm may contain either an X or a Y chromosome. Other animals have different methods of sex determination.

15 Figure 5.6a 44 + XY Parents 44 + XX or X Y Sperm 22 + X Egg 44 + XX or 44 + XY (a) The X-Y system Zygotes (offspring) 22 + XX 22 + X (b) The X-0 system

16 Figure 5.6b 76 + ZW 76 + ZZ (c) The Z-W system 32 (Diploid) 6 (Haploid) (d) The haplo-diploid system

17 A gene that is located on either sex chromosome is called a sex-linked gene. Genes on the Y chromosome are called Y-linked genes; there are few of these. Genes on the X chromosome are called X-linked genes. X chromosomes have genes for many characters unrelated to sex, whereas the Y chromosome mainly encodes genes related to sex determination.

18 X-linked genes follow specific patterns of inheritance. For a recessive X-linked trait to be expressed A female needs two copies of the allele (homozygous). A male needs only one copy of the allele (hemizygous). X-linked recessive disorders are much more common in males than in females.

19 Some disorders caused by recessive alleles on the X chromosome in humans. Colour blindness (mostly X-linked) NFL Color Rush Duchenne muscular dystrophy Hemophilia

20 Figure 5.7 X N X N X n Y X N X n X N Y X N X n X n Y Sperm X n Y Sperm X N Y Sperm Y X n Eggs X N X N X n X N Y Eggs X N X N X N X N Y Eggs X N X N X n X N Y X N X N X n X N Y X n X N X n X n Y X n X n X n X n Y (a) (b) (c)

21 Epistasis In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus. For example, in Labrador retrievers and many other mammals, coat colour depends on two genes. One gene determines the pigment colour (with alleles B for black and b for brown). The other gene (with alleles E for colour and e for no colour) determines whether the pigment will be deposited in the hair.

22 Figure 4.2 BbEe BbEe Eggs Sperm / 4 BE / 4 / be 4 Be / 4 be / 4 BE BBEE BbEE BBEe BbEe / 4 be BbEE bbee BbEe bbee / 4 Be BBEe BbEe BBee Bbee / 4 be BbEe bbee Bbee bbee 9 : 3 : 4

23 Polygenic Inheritance Quantitative characteristics are those that vary in the population along a continuum. Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype. Skin colour in humans is an example of polygenic inheritance.

24 Figure 4.3 Let A,B,C represent the presence of dark-skin alleles. Let a,b,c represent the absence of dark-skin alleles. AaBbCc AaBbCc Sperm / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 Eggs / 8 / 8 / 8 / 8 / 8 Phenotypes: / 64 6 / 64 5 / / 64 5 / 64 6 / 64 / 64 Number of dark-skin alleles:

25 Nature and Nurture: The Environmental Impact on Phenotype Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype. The norm of reaction is the phenotypic range of a genotype influenced by the environment. For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity.

26 Pedigree Analysis A pedigree is a family tree that describes the interrelationships of parents and children across generations. Inheritance patterns of particular traits can be traced and described using pedigrees.

27 Figure 4.5 Key Male Female Affected male Affected female Mating Offspring st generation 2nd generation 3rd generation Ww ww ww Ww Ww ww ww Ww Ww WW or Ww ww ww st generation 2nd generation FF or Ff 3rd generation Ff Ff ff Ff ff ff Ff ff Ff FF or Ff ff Widow s peak No widow s peak Attached earlobe Free earlobe (a) Is a widow s peak a dominant or recessive trait? b) Is an attached earlobe a dominant or recessive trait?