Genetics and Heredity Notes I. Background A. It was known for 1000s of years that traits were inherited but scientists were unsure about the laws that governed this inheritance. B. Gregor Mendel (1822-1884) was an Austrian monk who experimented with garden peas and developed the foundation of modern genetics. He noticed that peas had several traits and always showed only one of a pair rather than a blend which was previously believed. He crossed plants with different traits to see what the offspring would look like. C. Mendel found that no matter what combinations he tried, one trait always dominated and masked the other. It didn t matter if the trait came from the male or female parent. The traits were controlled by factors which were later known as genes. D. Mendel s Laws of Heredity 1. Inherited traits are controlled by genes that occur in pairs. These two versions are called alleles. For example, the gene that controls the color of the flowers in Mendel s peas has two alleles - purple and white. 2. An organism inherits an allele for each trait from each parent (2 alleles for each trait total) 3. One allele masks the presence of the other. Called the principle of dominance. Dominant (R) vs recessive (r) 4. Alleles separate during meiosis I. Called the law of segregation. E. Vocabulary 1. Homozygous - both alleles for a trait are the same 2. Heterozygous - the alleles for trait are different 3. Genotype - the actual genetic makeup for a trait 4. Phenotype - the way in which the genotype is expressed II. Monohybrid Cross A. Mendel found a 3:1 ratio in F 2 e.g., round seed wrinkled P RR x rr gametes R, R r, r Rr (all round) use two individuals as new Parents
P Rr x Rr gametes R, r R, r F 2 RR, Rr, Rr, rr (round, round, round, wrinkled; 3:1) B. A Punnett square can be used to show genotype, phenotype, and probability. e.g., heterozygous purple (Pp) x white (pp) p p P Pp Pp p pp pp 1 purple: 1 white e.g., two heterozygous tall plants (Tt) T t T TT Tt t Tt tt 3 tall: 1 short C. Test cross 1. Imagine that you have an organism showing a dominant phenotype. Is the individual homozygous or heterozygous? To be able to say for certain, a test cross is performed. 2. The unknown individual is crossed with a homozygous recessive individual. 3. The genotype of the unknown parent can be deduced from the appearance of the offspring. III. Dihybrid Cross A. Mendel wondered if traits always travelled together or if they were inherited separately.
e.g., if he crossed a yellow, round plant with a green, wrinkled plant would all the offspring be yellow, round or green, wrinkled or would some be yellow, wrinkled and some green, round B. He found a 9:3:3:1 ratio in the F 2 C. This showed that traits are inherited independently. e.g., YyRr x YyRrr YR Yr yr yr YR YYRR YYRr YyRR YyRr Yr YYRr YYrr YyRR Yyrr yr YyRR YyRr yyrr yyrr yr YyRr Yyrr yyrr yyrr 9 yellow, round: 3 yellow, wrinkled: 3 green, round; 1 green, wrinkled IV. The probability scale ranges from 0 to 1, where 0 means there is no chance the event will occur and 1 means the event will occur every time. Probability can be calculated using the equation: P = # total outcomes #correct outcomes A. Independent events - the outcome of previous events does not affect the outcome of future events. e.g., the chance of getting heads in a coin toss is ½; the chance of getting heads a second time is ½ B. Rule of Multiplication - The chance of two events occurring together is P 1 x P 2 C. Rule of Addition - The chance of either one of two possible outcomes occurring is the sum of the two individual probabilities. V. Cases of Non-Simple Dominance A. Incomplete Dominance 1. So far, offspring have showed the phenotype of one parent or the other. In some traits, the offspring have a phenotype which seems to be a blend of the two parents. 2. This means that heterozygotes will have a phenotype different from that of the two homozygous genotypes.
3. A 1:2:1 is characteristic of incomplete dominance. B. Codominance 1. A case in which two alleles are expressed at the same time. 2. The heterozygote phenotype appears to be a blend of the two homozygous phenotypes. 3. An example is roan cattle. A cross between a red bull and a white cow yields roan calves. They calves appear reddish in color but on closer inspection, they have both red and white hairs. In other words, BOTH alleles are expressed.. 4. A 1:2:1 is characteristic of codominance. C. Multiple alleles 1. Many genes actually have more than two alleles. 2. Remember that, although more than two alleles exist in the population, each individual only possesses two - one inherited from each parent. D. Epistasis 1. A gene at one locus alters the phenotypic expression of a gene at another locus. E. Polygenic inheritance 1. For some traits, an either-or result for phenotype does not exist. Rather, the phenotype differs along a continuum. 2. Examples are human height and skin color. VI. Human Genetic Disorders A. Recessively Inherited 1. Remember that genes code for proteins. An allele that causes a genetic disorder codes for a non-functional protein. Homozygous dominant (AA) and heterozygous (Aa) individuals are normal in phenotype because the one copy of the normal allele produces a sufficient quantity of the protein to prevent the disorder. 2. A homozygous recessive (aa) individual is unable to produce any of the protein in question. 3. Heterozygous individuals are said to be carriers - they have the recessive allele but do not show the recessive phenotype. 4. If the disorder is lethal before reproductive age, no homozygous recessives will reproduce. 5. In general populations, it is unlikely that two carriers of the same disorder will meet and mate. This probability increases, however, in matings between close relatives such as siblings or relatives. In these so called inbred matings, there is an increased risk that offspring will have a recessive genetic disorder. B. Dominantly Inherited Disorders 1. A lethal dominant allele is more rare because even heterozygotes are affected
C. Genetic counselling (i.e., die). If the disorder is lethal before reproductive age, the allele will not be passed on. The allele can be perpetuated in the population if it is late-acting. 1. Carrier recognition a. Because most children with genetic disorders are born to parents of normal phenotypes, it is important to identify parents who might be carriers before they reproduce. 2. Fetal testing - testing the fetus to determine the presence of any genetic disorders. Fetal testing gives parents the option of a. Amniocentesis - Beginning around the 14 th week of pregnancy, amniotic fluid is withdrawn from the uterus. Some disorders can be detected by the presence of certain chemicals in the fluid while fetal cells present in the fluid can be grown to be used for karyotyping to identify chromosomal abnormalities. b. CVS - chorionic villus sampling - A small amount of fetal tissue is removed from the placenta and the cells are used for karyotyping. The cells are growing quickly so results are available in 24h as opposed to several weeks as with amniocentesis. CVS can also be performed as early as the 8 th wekk of pregnancy. c. Ultrasound and fetoscopy - these techniques are used to produce and image of the fetus 3. New-born screening - some disorders can be detected at birth by testing the newborn baby. VII. Chromosomal Theory Main Points - it was noticed that the behaviour of chromosomes and that of genes were related. A. Chromosomes carry genes, the unit of hereditary. Both chromosomes and genes are both present in pairs in diploid cells. B. Homologous chromosomes separate and alleles segregate independently during meiosis. C. Fertilization restores chromosomes and genes to pairs. VIII. Morgan s Work A. Morgan was the first to work out that genes are located on chromosomes. B. He developed a different notation for genetic symbols. For example, the allele for the white eye mutation is symbolized by w, while the normal allele (called the wild type) is symbolized by w +. C. If the mutation is recessive, a lower case letter is used. Upper case is used for dominant mutant alleles. For example, curly wings is caused by a dominant allele and is symbolized by Cy, while normal wings is Cy +.
D. Sex-linkage 1. Morgan noticed a male with white eyes among many red-eyed males. 2. He crossed the white-eyed male with a red-eyed female and the were all red, as expected. The F 2 was 3 red:1 white. Only males, however, had white eyes and, among males, he noticed there was 1 red:1 white eyes. 3. A karyotype showed that males had a different chromosome from the females - the sex chromosome. Morgan reasoned that the gene for eye color must be located on the sex chromosomes and called the trait sex-linked. 4. Remember that a male always inherits a sex-linked trait from the female parent because the father always supplies the y chromosome. 5. X-inactivation - During early embryonic development, one X chromosome in each cell randomly becomes inactive. The chromosome condenses to a small spot near the nuclear membrane and is called the Barr body. E. Linked genes and Chromosome Mapping 1. The number of genes in a cell is far greater than the number of chromosomes so it stands to reason that each chromosome must carry many genes. These genes would tend to be inherited together and are called linked. 2. In some dihybrid crosses, Morgan found that most offspring had the same phenotypes as the parents, but other phenotypes were also observed. How could these other phenotypes arise? 3. Crossing over would occur more often between two genes as the distance between those two genes increased. If this relationship is linear, the frequency of crossing over could be used to determine the distance between two genes on the same chromosome. 4. One map unit is defined as 1% crossing over frequency. What is the maximum frequency for crossing over? If two genes are further apart than this value, how can they be mapped?