AUTOSOMAL DOMINANT TRAITS – Examining Family Trees and Calculating Probabilities Autosomal dominant traits are caused by possession a single copy of the dominant allele. When the dominant allele causes disease, only carriers are generally found to have the disease. Individuals with two copies of the dominant allele fail to survive. Achondroplasia (a common type of dwarfism) is an example, although many cases of achondroplasia are spontaneous. I II III IV affected male affected female Examine the family tree above showing presence or absence of an autosomal dominant trait. 1. What can you conclude about the genes possessed by the parents in generation I. Male genotype: __________ Female genotype: __________ 2. Determine the probability the couple in generation I having a child with the autosomal dominant disease. 3. The probability of the couple in generation I having an unaffected child is __________. 4. The probability of the couple in generation I having an affected child is __________. 5. Determine the probability of the couple in generation I having a child who is a carrier? __________. 6. Examine the family tree across the generations. What is your observation with respect to the presence of the disease in each generation?
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Autosomal Recessive Recessive traits are those which are expressed only if there are no dominant alleles present. In the case of autosomal recessive traits, only the homozygous recessive genotype will have the recessive phenotype. For recessive traits, individuals who are heterozygous are referred to as carriers. This is because there is not a phenotypic expression of the trait, but the allele does exist and can be passed onto offspring. Cystic fibrosis is an autosomal recessive disease. The cystic fibrosis allele causes an abnormal chloride ion channel. This abnormally functioning channel results in high levels of production of viscous mucus, which can lead to increased respiratory infections. "F" represents the normal allele. "f" represents the cystic fibrosis allele. Answer the following questions about a cross (or mating) between a father who is "ff" and a mother who is "FF". What is the father's genotype (heterozygous, homozygous dominant, or homozygous recessive)? What is the mother's genotype (heterozygous, homozygous dominant, or homozygous recessive)? What is the father's phenotype (affected, unaffected, carrier)? What is the mother's phenotype (affected, unaffected, carrier)? What are the potential gametes produced by the father? What are the potential gametes produced by the mother? Complete a Punnett square for a cross between these two individuals. Parent #1 → Parent #2 ↓ What is the predicted ratio of phenotypes for the cross described? Now, answer the following questions about a cross (or mating) between a father who is "Ff" and a mother who is "Ff" for cystic fibrosis. What is the father's genotype (heterozygous, homozygous dominant, or homozygous recessive)? What is the mother's genotype (heterozygous, homozygous dominant, or homozygous recessive)? What is the father's phenotype (affected, unaffected, carrier)? What is the mother's phenotype (affected, unaffected, carrier)? What are the potential gametes produced by the father? What are the potential gametes produced by the mother? Complete a Punnett square for a cross between these two individuals. Parent #1 → Parent #2 ↓ What is the predicted ratio of phenotypes for the cross described?
Madhur L.
Achondroplasia is a dominant form of dwarfism in humans. Heterozygous individuals exhibit the dwarfism trait, while homozygous dominant embryos die before birth. Suppose that an individual with Achondroplasia is expecting a child with an individual of normal height. What are the chances that their child will exhibit the dwarfism trait? In guinea pigs, rough coat is a dominant gene and a smooth coat is the recessive allele. A rough-coated male is mated with two rough-coated females. Female A, after three matings, has produced 14 young, all rough coated. Female B, after three matings, has produced nine young; seven of the offspring are rough coated and two are smooth coated. What are the genotypes of the three parent animals? Maple syrup urine disease is a rare inborn error of metabolism. If untreated, affected children die soon after birth. The disease tends to recur in the same family, but the parents of the affected individuals are always normal. What does this information suggest about the transmission of the disease?
Adi S.
Suppose in humans an autosomal gene causes dwarfism in one homozygous state (aa), normal height in the heterozygous state (Aa), and is embryonic lethal in the other homozygous state (AA). A separate gene found on another chromosome causes albinism; an autosomal recessive disorder. Albino individuals have the genotype aa. Rick and Lori, who are both heterozygous for both genes (Aa), just had a son named Carl. What is the probability of Carl being of normal height with normal skin pigmentation? Remember that one of the homozygous genotypes at the locus is lethal. Therefore, the correct ratio of offspring at this locus is 2 normal : 1 dwarf. Calculate the probability again, eliminating all offspring from your calculations.
Sri K.
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