lecture 12 for BGYA01

11 Pages
Unlock Document

University of Toronto Scarborough
Biological Sciences
Clare Hasenkampf

BGYA01 Lecture 12 October 23, 2007 Last class we started our consideration of transmission genetics the study of how genes are passed from one generation to the next. We looked at the Law for segregation of alleles and independent assortment. We also looked at how you can use your knowledge of these two laws to create Punnett Squares that allow you to predict the outcomes of particular mating. If you have attempted the assigned problems you will see that truly understanding these two laws and their consequences (combine how to correctly use a Punnett Square or probability theory) is very powerful. Last class we saw how genotypes give rise to phenotypes WHEN the alleles examined have only simple dominance recessive relationships. Today we are going to see some things that complicate our predictions. - for some genes both parents dont have two versions of each gene - some alleles are not recessive or dominant. - Sometimes genes interact Thus in controlled genetic crosses you dont always get 3:1 phenotypic ratio in the F of 2 a monohybrid cross, and you dont always get a 9:3:3:1 phenotypic ratio in the F of a 2 self-crossed dihybrid. Lets explore this further. Last class we look at the results of a monohybrid cross (of dominantrecessive alleles) in which the F w1re selfed to give rise to the F . 2 In the F2that is produced from a self cross of the F , 1e expect the Mendelian genotypic ratio of 1:2:1 (ss:S s:SS) This 1:2:1 genotypic ratio was obtained because of Segregation of alleles and random fertilization. In his crosses all offspring received one version of the gene from each parent, and each parent had two versions of the gene. These classical Mendelian results occur if the genes in question are autosomal genes. In fact, up to now, in the crosses we have looked at we have been considering the inheritance of autosomal genes. What are autosomal genes? Autosomal genes are located on the autosomes. What are autosomes? Autosomes are the chromosomes of the organism for which the chromosome pairs are the same in BOTH sexes. www.notesolution.com 1In humans all of us have 22 pairs of homologous chromosomes (chromosome pairs 1- 22) that we call autosomes. Males and females both have 2 of each autosome Figure 9.15, page 195. X-LINKED GENES Sex chromosomes are specific chromosomes that are different in males and females, and are important in determining the sex of the individual. (Figure 9.15, page 195 the X and Y chromosome) Not all eukaryotic organisms have sex chromosomes. Some plants, and most higher animals DO have sex chromosomes In humans we have sex chromosomes. Normal women have two X chromosomes. Normal men have one X chromosome, and one Y chromosome. The X and Y chromosomes are the sex chromosomes in humans. SEX DETERMINATION IN HUMANS The sex chromosomes determine the sex of an individual. In humans the female body plan is the default plan. All of us start out using the female body plan. XX individuals stay with this plan. Lack of a Y means female. In humans the Y chromosome determines maleness. The Testis Determining Factor TDF gene is on the Y chromosome. This gene turns on very early in development, and causes a testis to develop, the testis produces the hormone testosterone, then the female body plan will be blocked and the male body plan will develop. Now lets get back to inheritance. Lets look briefly at the human X and Y chromosomes. Pseudoautosomal region Y X TDF gene X-linked genes all along the non-psueodoautosomal region of the X chromosome The very top portion of the X and Y chromosome is homologous and is called the pseudo-autosomal region (because genes in this region behave like autosomal genes since the pseudoautosomal region is the same in males and females have). www.notesolution.com 2
More Less

Related notes for BIOA01H3

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.