Blueprint of Life Topic 12: Sex Linked Genes

1. Blueprint of LifeTopic 12: Sex Linked Genes Biology in Focus, HSC Course Glenda Childrawi, Margaret Robson and Stephanie Hollis

2. DOT POINT(s) • describe the inheritance of sex-linked genes, and alleles that exhibit co‑dominance and explain why these do not produce simple Mendelian ratios • describe the work of Morgan that led to the understanding of sex linkage

3. Introduction Mendelian ratios of inheritance apply only in situations where conditions are similar to those studied by Mendel. However, if genes do not assort independently or do not show dominance, Mendel’s ratios are not obtained. homeschoolersresources.blogspot.com

4. Introduction Two examples where these deviations from Mendel’s ratios have been seen are in sex-linked inheritance and co-dominance, which is what we are going to talk about today. en.wikipedia.org

5. Sex-linkage Every cell in the human body contains 23 pairs of chromosomes: 22 pairs of autosomes (chromosomes that code for general traits within the body) and 1 pair of sex chromosomes ghr.nlm.nih.gov

6. Sex-linkage Sex chromosomes carry genes that determine the sexual characteristics of a person and therefore influence whether they are male or female. Sex chromosomes in an individual may differ from each other in size and shape (in contrast to autosomes, where homologous chromosomes all have the same appearance). www.psmicrographs.co.uk

7. Sex-linkage In humans: • Females have a pair of similar sex chromosomes— their genotype is represented as XX (described as homogametic). • Males appear to be different (heterogametic) and their genotype is represented as XY. In human males, one sex chromosome is a typical X chromosome, but the other is shorter and seems to be missing part of its ‘arms’, resulting in a ‘Y’ shape leavingbio.net

8. Sex Determination The offspring of most animals have an equal (50%) chance of being male or female. This is determined by the following mechanisms during the life cycle: ■ the segregation of sex chromosomes during meiosis ■ the transfer of one sex chromosome to each gamete ■ the fusion of gametes during fertilisation. www.sodahead.com

9. Sex Determination During meiosis, the sex chromosomes segregate, just like any other homologous pair of chromosomes, and only one of each chromosome pair passes into a gamete. www.biology.iupui.edu

10. Sex Determination Since a female has 44 autosomes + XX, when the chromosome number is halved during gamete formation, each female gamete (egg cell) receives 22 autosomes + X. In males, half the gametes (sperm cells) receive 22 autosomes + X and the other half receive 22 autosomes + Y. . It is therefore the male gamete in humans that determines the sex of the offspring. www.bbc.co.uk

11. Sex Determination Fertilisation follows and the combination of sex chromosomes in the zygote dictates the sex of the child: a zygote that inherits an X chromosome from both the mother and father will be female (XX). A zygote that receives an X chromosome from the mother and a Y chromosome from the father will be male (XY). In humans, Y carries the testis-determining gene and therefore, if present, the Y assures the child will be male. toppersguide17.blogspot.com

12. Sex Determination In some cases, non-disjunction of chromosomes occurs during meiosis (the chromatids do not separate), resulting in the incorrect number of chromosomes in the offspring (e.g. 45 or 47). If this occurs in the sex chromosomes and only one X chromosome is present, the child will be female. If the individual is XXY, it will be male. www.zazzle.ca

13. Sex Determination Turner Syndrome occurs when only one X chromosome is present. There are characteristic physical abnormalities, such as short stature, swelling, broad chest, low hairline, low-set ears, and webbed necks. Girls with Turner syndrome typically experience gonadal dysfunction (non-working ovaries), which results in amenorrhea (absence of menstrual cycle) and sterility. www.doctortipster.com

14. Sex Determination Concurrent health concerns are also frequently present, including congenital heart disease, hypothyroidism (reduced hormone secretion by the thyroid), diabetes, vision problems, hearing concerns, and many autoimmune diseases. Finally, a specific pattern of cognitive deficits is often observed, with particular difficulties in visuospatial, mathematical, and memory areas. en.wikipedia.org

15. Sex Determination XYY syndrome is an abnormal number of the sex chromosomes in which a human male receives an extra Y-chromosome, giving a total of 47. This produces a 47,XYY karyotype, which occurs in 1 in 1,000 male births. Some medical geneticists question whether the term "syndrome" is appropriate for this condition because its clinical phenotype is normal and the vast majority (an estimated 97% in Britain) of 47,XYY males do not know their karyotype. powerofthegene.com

16. Sex-linked Genes The larger sex chromosome (X in humans) may also carry a few genes that code for non-sexual body characteristics. These genes are termed sex-linked genes, since they are physically linked to the sex chromosome and are inherited together with the sexual traits. www.biologycorner.com

17. Sex-linked Genes Sex-linked genes in males and females tend to differ in their inheritance patterns, since males lack one X chromosome and therefore have only one allele for each sex-linked gene, rather than a pair of alleles (as is present in females). feistyhome.phpwebhosting.com

18. Sex-linked Genes Mendel’s experiments did not show sex-specific effects and so sex-linked inheritance shows a deviation from Mendel’s expected ratios. (Today, some scientists wonder whether Mendel was just fortunate in selecting only nonlinked traits, or if he perhaps simply disregarded results that did not ‘fit’ his ratios.) chubbyriceball.wordpress.com

19. Thomas Morgan When Thomas Hunt Morgan, an American cytologist, began his now famous genetics experiments in 1910, the paradox that faced scientists was: If chromosomes are the basis of inheritance, why do the number of traits that separate during meiosis exceed the number of chromosomes? bio.as.uky.edu

20. Thomas Morgan Morgan was initially sceptical about aspects of both Mendelianinheritance and the chromosome theory and so it is ironic that it was his experiments on the fruit fly, Drosophila melanogaster, that provided the significant evidence needed by scientists before they could finally accept Sutton and Boveri’schromosome theory of inheritance (that more than one factor (trait) was present on each chromosome). www.taxateca.com

21. Thomas Morgan Morgan’s experiments showed without any doubt that: ■ the gene for eye colour in fruit flies is located on the X chromosome, and ■ hereditary factors can be exchanged between the X chromosomes of an individual. www.brain.riken.go.jp

22. Thomas Morgan The results of Morgan’s studies led to a greater understanding of how genes are arranged on chromosomes and how genetic material can be exchanged (during crossing over) in meiosis. He was awarded the Nobel Prize for Physiology or Medicine in 1933. www.nobelprize.org

23. Thomas Morgan The Nobel Prize is a set of annual international awards bestowed in a number of categories by Scandinavian committees in recognition of cultural and/or scientific advances. The will of the Swedish philanthropist inventor Alfred Nobel established the prizes in 1895. The prizes in Physics, Chemistry, Physiology or Medicine, Literature, and Peace were first awarded in 1901. en.wikipedia.org

24. Morgan’s Experiment Morgan was working with Drosophila melanogaster (fruit flies), which normally have red eyes. He discovered a mutant male fruit fly that had white eyes. www.nobelprize.org

25. Morgan’s Experiment Morgan began with a series of crosses in the typical Mendelian sequence to determine if the gene for white eyes would show a Mendelian pattern of inheritance: ■ Cross 1: He cross-bred pure-breeding parents to obtain F1 hybrid offspring. Morgan crossed a white-eyed male and a pure-bred (homozygous) redeyed female. Hand out Diagram

26. Morgan’s Experiment Cross 2: He then crossed the F1 hybrid offspring to obtain the F2 generation. Morgan was expecting a Mendelian3:1 ratio but, surprising to him, his results showed something different—more than 80% of the flies had red eyes and less than 20% had white eyes; in addition, most flies with white eyes were male. At first, he thought that perhaps female flies could not have white eyes. www.gizmodo.com.au

27. Morgan’s Experiment ■ Cross 3: He performed a typical ‘test cross’ to investigate this hypothesis. He crossed a white-eyed male with a hybrid red-eyed female. His results showed in the F2 that both females and males could have white eyes. He would have to come up with another hypothesis to explain the unexpected ratios in his second cross. www.gizmodo.com.au

28. Morgan’s Experiment He arrived at his next (and this time correct) hypothesis that the white eye characteristic is ‘sex limited’ (Morgan’s words) and is carried on the X chromosome. He followed this with subsequent genetic crosses that proved that red eyes were in fact sex-linked (carried on a sex chromosome). library.thinkquest.org

29. Explaining Symbols Since the inheritance of a sex-linked gene is being studied, the alleles of that gene (e.g. R = red eye colour, r = white eye colour) and the type of chromosome on which it is carried (X or Y) must be shown in the genetic cross. The letters R and r are assigned in keeping with the symbols used to show dominant and recessive alleles of a gene. www.iq.poquoson.org

30. Explaining Symbols These are then written as a superscript to the chromosome on which they are attached: www.gizmodo.com.au

31. Morgan’s Experimental Significance Morgan’s work was significant because it provided evidence for the chromosomal basis of sex-linkage: his experiments showed that the gene for eye colour is linked to other genes that determine the sex of Drosophila melanogaster —that is, that the gene for eye colour is sex-linked. This paved the way for many more experiments to show linkage on all chromosomes. suite101.com

32. Human Example: Haemophilia The gene that codes for a certain bloodclotting factor is normally situated on the X chromosome. In males who lack an allele of this gene, haemophilia results as they have no similar gene on the Y chromosome to compensate and so they are unable to produce the clotting factor. research.fuseink.com

33. Human Example: Haemophilia In females lacking one allele of this gene, another may be present since they have two X chromosomes. If the allele present is not defective, they do not have the disease haemophilia and are said to be ‘carriers’ of the disease, since they can pass their defective gene on their X chromosome to their sons (who would be afflicted) or to their daughters (who could then be carriers). simple.wikipedia.org

34. Human Example: Haemophilia Daughters will be affected only if they inherit a pair of defective alleles from both parents—one defective allele on each of their X chromosomes. Colour blindness in humans is also an X-linked trait. Students who study the HSC Option: Communication will need to know about colour blindness. www.ehc.eu

35. Activity/Homework -Students to complete The work of Tomas Hunt Morgan Worksheet (CD) a

36. Extension -Handout 12.1.4 Additional Information of Sex linked genes in humans a