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Other inheritance patterns. Warm Up. For the cross AABBCCDd x AAbbCcDd, what is the probability that an offspring will be AABbCcDd? A.) 1/16 B.) 1/8 C.) 1/4 D.) 3/8 E.) 1/2. Epistasis. One gene controls the expression of another gene. Frequently occurs in pigmentation.
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Warm Up • For the cross AABBCCDd x AAbbCcDd, what is the probability that an offspring will be AABbCcDd? • A.) 1/16 • B.) 1/8 • C.) 1/4 • D.) 3/8 • E.) 1/2
Epistasis • One gene controls the expression of another gene. • Frequently occurs in pigmentation. • One gene turns on (off) the production of pigment, while the second gene controls the amount of pigment or the color. • If the first gene codes for no pigment, the expression of the second gene has no effect.
Epistasis example: • Fur in mice: • Gene 1 codes for presence or absence of pigment • Gene 2 codes for the color (brown or black) • C = pigment; B = black • What would the genotypes for black fur look like? (Hint: there are 4 possible genotypes for black fur) • CCBB, CCBb, CcBB, CcBb • What about CCbb and Ccbb?
Epistasis example continued… • What happens when “cc” is inherited? • No pigment is produced and the fur is white, regardless of the color encoded by the B allele.
Pleiotrpoy • One gene has more than one phenotypic expression. • Example: • In pea plants, the gene that expresses the round or wrinkled texture of seeds also influences the phenotypic expressions of starch metabolism and water absorption.
How? • The allele for round seeds codes for a greater conversion of glucose to starch than does the allele for wrinkled seeds. • In wrinkled seeds there is more unconverted glucose. • Higher concentration of glucose = increased osmotic gradient = more absorption of water. • So, immature wrinkled seeds have more water. • However, when these seeds mature, they dehydrate resulting in the wrinkled appearance. • In contrast, round seeds absorb less water, so they lose less water during dehydration, therefore wrinkle less.
Pleiotropy in humans • Sickle cell anemia • An allele incorrectly codes for hemoglobin. • Abnormal hemoglobin caused the red blood cell to become sickle shaped. • In response to the shape change, the rbcs do not flow through the capillaries freely and O2 delivery is inadequate. • This causes a general breakdown in the body, including damage to heart, lungs, kidneys, brain. • This promotes disorders such as anemia, pneumonia, heart and kidney failure, bone abnormalities and impaired mental functioning.
Polygenic Inheritance The opposite of pleiotrophy (one gene, many characteristics) is polygenic inheritance which is the case where many genes act on a single characteristic. For example: skin color is determined by at least 3 separately inherited genes. Variations of the genotype of these individuals produces all of the varieties of skin color we see.
Sex Determination Whether or not a person is male or female is determined from their genotype: XX is female; XY is male. In humans, the father determines the sex of the baby. The chance of being a male or female is 50/50. Half of the sperm will inherit a Y, the other half will inherit the X.
Sex Determination and the Y Chromosome The Y chromosome contains a region (SRY gene) which codes for proteins that induce the gonads to form testes. In the absence of this protein, the gonads form ovaries.
Sex Determination and the X Chromosome Inheriting an X chromosome from dad will give a female 2 X chromosomes. Only one functions within the cell, the other is inactivated. It becomes a Barr body. The Barr body becomes reactivated in gametes so all of them have an active X chromosome when produced.
X Inactivation This process is a totally random event and occurs independently in embryonic cells at the time of X inactivation. Females consist of a mosaic of active X genes--those derived from the father and those derived from the mother.
X Inactivation As the embryo continues to divide mitotically, the we now have groups of cells with active X chromosomes derived from the mom, and active X chromosomes derived from dad. If a female is heterozygous for a sex linked trait, approximately 1/2 of the cells will express one gene, and 1/2 will express the other gene.
X Inactivation and Mosaicism X inactivation can be seen in calico cats. It can also be seen in a sweat gland disorder.
Linked Genes • If genes are on different chromosomes, the genes segregate independently from one another (Law of Independent Assortment) • However, if the genes are on the same chromosome, they cannot segregate independently because they are physically connected. • Genes that are linked are usually inherited together.
Nondisjunction Normally, in meiosis, the chromosomes are distributed without fail and the numbers of chromosomes remains the same throughout the generations. Occasionally, chromosomes don’t get separated properly in meiosis I or II. Some gametes fail to receive a copy of a chromosome; others receive 2 copies.
Aneuploidy When nondisjunction occurs and is followed by fertilization, a situation arises where an abnormal number of chromosomes are present in the developing organism.
Aneuploidy A cell with triple the number of a chromosome is known as trisomy. Trisomy 21 or Down Syndrome is an example. Having only one copy of the chromosome produces a situation known as monosomy.
Nondisjunction and Mitosis Nondisjunction occurs in mitosis too. If it occurs very early on, then the organism, if it survives, will likely have a large number of phenotypic abnormalities.
Chromosomal Alterations 1. Deletion--a gene or base pair is lost. 2. Duplication--a segment of DNA gets repeated. 3. Inversion--occurs when a chromosomal fragment flip-flops and reattaches to the original chromosome. 4. Translocation--occurs when a fragment from one chromosome is lost and becomes attached to another chromosome.
Deletions and Duplications Occur most often during meiosis because of crossing over.
Duplications and Translocations These don’t alter the balance of genes, but the order on the chromosome is disrupted. This effects the neighboring genes and the expression of the duplicated/translocated genes. They are usually lethal.
Non-Lethal Disruptions Aneuploidy--Down Syndrome. It isn’t a duplication event, but essentially is like a duplication event.
Aneuploidy Klienfelter Syndrome-XXY males. Have male sex organs but their testes are small and they are sterile. Also have other feminine characteristics such as gynecomastia, fat deposits in hips, no male secondary sex characteristics.. They can be of normal intelligence, but some often exhibit some metal impairments.
Aneuploidy XXX females cannot be distinguished from any other female except by karyotype. XO are females with Turner’s syndrome. It is the only known monosomy in humans. They are sterile because their sex organs don’t mature, can develop 2° sex characteristics with hormone treatment.
Extranuclear Genes These are the genes found on the chromosomes of organelles such as mitochondria and chloroplasts. These are derived from the mother and replicate themselves. They code for the proteins and RNA that they use to perform their particular functions.
