Sex linked genes

Why are some characteristics ‘sex linked’? The X-chromosome 23 carries 1098 genes Over 100(recessive) genes for genetic disorders have now been mapped to the X-chromosome 23. The human X-chromosome is much smaller, and only carries a few genes

Sex-linked inheritance

Red-green colour blindness test......

Genetics of colour blindness 0.4% of women are colour blind 7% of men are colour blind In colour blindness, green or red cones work poorly, or not at all… Genes for green and red cones are close to each other on the X-chromosome Why is ‘blue blindness’ rare?

Haemophilia Recessive, sex linked disease which causes abnormal blood clotting Individuals are at risk of severe bleeding Many die from brain haemorrhages Type A: Lack of Factor VII: 80% of cases of haemophilia Type B: Lack of Factor IX: 20% of cases

Haemophilia in the Royal families of Europe Queen Victoria has 9 children (4 sons, 2 daughters). She was a carrier for the Haemophilia gene 1 of 4 sons developed haemohilia (died at age 30) 2 of 5 daughers were carriers (married and moved to Spain, Russia, Germany)

Diseases linked to the 23rd chromosome More than 100 sex-linked genetic disorders have been linked to the X-chromosome of chromosome 23 in humans, including: Haemophilia Duchenne Muscular dystrophy Colourblindness Melanoma X-linked severe combined immunodeifciency X-inactivation centre

Why are sex-linked diseases more common in males? Males have only 1 X chromosome; thus all X-linked alleles will be phenotypically expressed in males, even if they are recessive… Females must have BOTH copies of the allele to be recessive, in order to express the gene phenotypically Thus the recessive phenotype of a sex-linked genetic disorder is much more common in males Sex-linked genes can move from fathers to their daughter, where the daughters will be unaffected carriers, but pass the gene to their sons…

Inherited Diseases

Autosomal Dominant Disorders Neurofibromatosis Huntington’s Disease

Autosomal dominant disease: Huntingdon’s Disease

Can we test for Huntingdon’s Disease? Pre-natal testing (amniocentesis or chorionic villus sampling) Genetic testing after birth

Autosomal Recessive Disorders Tay Sachs Cystic Fibrosis

Genetic Diseases

Inherited diseases Phenylketonuria Sickle Cell disease Cystic fibrosis

Cystic Fibrosis Phenylketonuria Sickle Cell Disease

Inherited diseases

Genetic Testing (1) How to read a chromosome

Genetic Testing (2) There are various types of genetic testing: Prenatal Newborn Screening (e.g. newborn test for PKU) Carrier diagnosis for couples with a family history of disease Late-onset disorders can be tested at any time DNA fingerprinting

Genetic Testing and Counseling Diagnosis of carriers Biochemical (i.e. Tay-Sachs) Genetic (i.e. Huntington’s disease) Ethical considerations Fetal diagnosis Amniocentesis Chorionic villi sampling

Chorionic villi sampling Chorionic villus sampling (CVS) is the removal of a small piece of tissue (chorionic villi) from the uterus during early pregnancy to screen the baby for genetic defects.

A son with cystic fibrosis (autosomal recessive) is born to a couple who appear to be normal. What are the chances that any child born to this couple will have cystic fibrosis? And the Answer is….. 25%

In humans the allele for short fingers is dominant over that for long fingers. If a person with short fingers who had one parent with long fingers reproduces with a person having long fingers, what are the chances of each child having short fingers? And the Answer is….. 50%

Homework: Complete the remain questions on the genetics problem set and do your genetics mini project.

Combining multiplication and addition rules to solve complex problems An organism with the genotype BbDD is mated with one with the genotype BBDd. Assuming independent assortment of these two genes, what is the probability that you will get a BBDD offspring? ¼ BBDD