Essential knowledge 3 A 3 The chromosomal basis

Essential knowledge 3. A. 3: The chromosomal basis of inheritance provides an understanding of the pattern of passage (transmission) of genes from parent to offspring.

Probability and Genetics �Rules of probability can be applied to analyze passage of single gene traits from parent to offspring. �Two basic rules of probability are helpful in solving genetics problems: the rule of multiplication (or the rule of and) and the rule of addition (or the rule of or). �

Rule of multiplication is that the probability that independent events will occur simultaneously is the product of their individual probabilities. For example: �Question: �In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability that the offspring will be homozygous recessive? �Answer: �Probability that an egg from the F 1 (Pp) will receive a p allele = 1/2. �Probability that a sperm from the F 1 will receive a p allele = 1/2. �The overall probability that two recessive alleles will unite, one from the egg and one from the sperm, simultaneously, at fertilization is: 1/2 X 1/2 = 1/4.

Rule of addition is that the probability of an event that can occur in two or more independent ways is the sum of the separate probabilities of the different ways. For example: �Question: �In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability of the offspring being a heterozygote? �Answer: �There are two ways in which a heterozygote may be produced: the dominant allele (P) may be in the egg and the recessive allele (p) in the sperm, or the dominant allele may be in the sperm and the recessive in the egg. Consequently, the probability that the offspring will be heterozygous is the sum of the probabilities of those two possible ways: �Probability that the dominant allele will be in the egg with the recessive in the sperm is 1/2 X 1/2 = 1/4. �Probability that the dominant allele will be in the sperm and the recessive in the egg is 1/2 X 1/2 = 1/4. �Therefore, the probability that a heterozygous offspring will be produced is 1/4 + 1/4 = 1/2.

�Segregation and independent assortment of chromosomes result in genetic variation. �Segregation and independent assortment can be applied to genes that are on different chromosomes.

Linked Genes are found on the same chromosome �Genes that are adjacent and close to each other on the same chromosome tend to move as a unit �the probability that they will segregate as a unit is a function of the distance between them(Crossover frequencies).

�The pattern of inheritance (monohybrid, dihybrid, sex -linked, and genes linked on the same homologous chromosome) can often be predicted from data that gives the parent genotype/phenotype and/or the offspring phenotypes/genotypes. �Be prepared to work any of the above type of problems on the AP Exam.

Linked �In D r o s o p h i l a t he genes A B D are linked. You cross a strain that is homozygous for d with a strain that is homozygous for ab. Below are the phenotypes and progeny of a test cross. Outline the cross and determine the cross-over frequencies and the map for the genes. Phenotype. Number ABd 389 ab. D 413 ABD 60 abd 68 a. Bd 29 Ab. D 34 Abd 3 a. BD 4

Certain human genetic disorders can be attributed to the inheritance of single gene traits or specific chromosomal changes, such as nondisjunction. �Sickle cell anemia �Tay-Sachs disease �Huntington’s disease �X-linked color blindness �Trisomy 21/Down syndrome �Klinefelter’s syndrome

Sickle cell anemia �is an autosomal recessive genetic blood disorder with overdominance, characterized by red blood cells that assume an abnormal, rigid, sickle shape. �Sickling decreases the cells' flexibility and results in a risk of various complications. �The sickling occurs because of a point mutation (substitution) in the hemoglobin gene.

Tay-Sachs disease �Another example of a substitution point mutation. �Tay–Sachs disease is a rare autosomal recessive genetic disorder. �In its most common variant (known as infantile Tay– Sachs disease), it causes a progressive deterioration of nerve cells and of mental and physical abilities that commences around six months of age and usually results in death by the age of four.

Huntington’s disease �a neurodegenerative genetic disorder that affects muscle coordination and leads to cognitive decline and psychiatric problems. �The disease is caused by an autosomal dominant mutation in either of an individual's two copies of a gene called Huntingtin, which means any child of an affected person typically has a 50% chance of inheriting the disease.

X-linked color blindness

Trisomy 21/Down syndrome

Klinefelter’s syndrome � a genetic disorder in which there is at least one extra X chromosome to a standard human male karyotype, for a total of 47 chromosomes rather than the 46 found in genetically normal humans � As babies and children, XXY males may have weaker muscles and reduced strength. As they grow older, they tend to become taller than average. They may have less muscle control and coordination than other boys their age. � During puberty, the physical traits of the syndrome become more evident; because these boys do not produce as much testosterone as other boys, they have a less muscular body, less facial and body hair, and broader hips. As teens, XXY males may have larger breasts, weaker bones, and a lower energy level than other boys. � By adulthood, XXY males look similar to males without the condition, although they are often taller.

�d. Many ethical, social and medical issues surround human genetic disorders. �To foster student understanding of this concept, instructors can choose an illustrative example such as: � • Reproduction issues � • Civic issues such as ownership of genetic information, privacy, historical contexts, etc.