HUMAN GENETICS

Objectives 2. Discuss the relationships among chromosomes, genes, and DNA.

2.8 Examine incomplete dominance, alleles, sex determination, and sex-linked traits in the context of human genetics.

2.9 Discuss several human genetic disorders such as hemophilia, sickle-cell anemia, Down's syndrome, and Tay-Sach's disease.

2.10 Discuss the similarities and differences between sex chromosomes and somatic chromosomes.

Sex Determination•Somatic chromosomes – any chromosomes which are not sex chromosomes

•Sex Chromosomes – Chromosomes that differ in males and females of the same species; combination of sex chromosomes determine the sex of the offspring. In most mammals sex chromosomes consist of a matching pair of homologous chromosomes in females, (XX) and a partially matching pair in male (XY). The X chromosomes are the larger of the two, the Y chromosome is a smaller chromosome.

•During meiosis the two sex chromosomes are separated into the four daughter cells. If fertilization occurs the embryo will receive on X chromosome from the mother and either an X or Y chromosome from the father. The chromosome from the father will determine the sex of the offspring depending on which chromosome is passed down.

Sex Linked Traits•Autosomal Inheritance – inheritance of alleles located on autosomal (non-sex chromosomes). Both male and females will be affected equally

•Sex-linked – describes an allele that is found on one of the sex chromosomes, X or Y, and when passed on to offspring is expressed.

•X-linked – phenotypic expression of an allele that is found on the X chromosome

•Y-linked – phenotypic expression of an allele that is found on the Y chromosome

Sex Linked Traits•If a male inherits an X chromosome with a sex linked trait from a mother who carries the recessive allele, he will express the disorder because the chromosome cannot mask the effects of that allele

•The male cannot inherit an x-linked disorder from his father, since a father passes on a Y chromosome to a son.

•A female must inherit two copies of the recessive gene – one on each X chromosome – in order to express the disorder

•Y-linked disorders also exist and are passed from father to son.

Sex Linked Punnett Square•A woman who carried the allele for colour blindness has a child with a man who is colour blind. What are the possible genotypes and phenotypes of their offspring?

Assignement

•Genetic Disorder Brochure

Hemophilia •Hemophilia is carried on the X chromosome. It is called an X linked genetic disorder. A women who is a carrier for hemophilia has the genetic mutation on one of her X chromosomes. She will have another non mutated X chromosome that will usually somewhat compensate for the defect in the other. A man who has hemophilia has the genetic mutation on his only X chromosome. He does not have another X chromosome to compensate for the defect so he will have hemophilia. 2/3rds of the cases of hemophilia are inherited. The other 1/3 occur with no family history and are called spontaneous genetic mutations. The spontaneous genetic mutation may either occur in a female fetus or a male fetus. The female fetus will become a carrier of hemophilia. The male fetus will become a person affected by hemophilia.

Sickle-Cell Anemia•Sickle cell disease is an inherited disorder in which red blood cells (RBCs) are abnormally shaped. This abnormality can result in painful episodes, serious infections, chronic anemia, and damage to body organs.

•Sickle cell disease is an autosomal recessive disorder, which means that a couple is only at risk of having a child with sickle cell disease if both reproductive partners are carriers. When both partners are carriers of sickle cell disease, there is a 25% chance of each child having the disease.

Down's Syndrome•Down syndrome is typically caused by what is called nondisjunction. Nondisjunction happens when a pair of chromosomes fails to separate during egg (or sperm) formation. When that egg unites with a normal sperm to form an embryo, the embryo ends up with three copies of chromosome 21 instead of the normal two. The extra chromosome is then copied in every cell as the baby develops.

•Interestingly, nondisjunction events seem to occur more frequently in older women. This may explain why the risk of having a baby with Down syndrome is greater among mothers age 35 and older.

Tay-Sachs Disease•Tay-Sachs disease is a rare inherited disorder that progressively destroys nerve cells (neurons) in the brain and spinal cord.

•Tay-Sachs disease results from defects in a gene on chromosome 15 that codes for production of the enzyme Hex-A. We all have two copies of this gene. If either or both Hex-A genes are active, the body produces enough of the enzyme to prevent the abnormal build-up of the GM2 ganglioside lipid. Carriers of Tay-Sachs - people who have one copy of the inactive gene along with one copy of the active gene - are healthy. They do not have Tay-Sachs disease but they may pass on the faulty gene to their children.

•Carriers have a 50 percent chance of passing on the defective gene to their children. A child who inherits one inactive gene is a Tay-Sachs carrier like the parent. If both parents are carriers and their child inherits the defective Hex-A gene from each of them, the child will have Tay-Sachs disease. When both parents are carriers of the defective Tay-Sachs gene, each child has a 25 percent chance of having Tay-Sachs disease and a 50 percent chance of being a carrier.