2. Inheritance patterns in humans Achondroplasia is an autosomal dominant disorder that affects skeletal growth and results in a greatly reduced stature. The presence of the achondroplasia allele causes a condition sometimes referred to as "dwarfism" (although it is important to note that it is not the only cause of dwarfism; many other genes and biological factors can cause dwarfism as well). Achondroplasia occurs only when a person is heterozygous for the allele. For reasons still unknown, being homozygous dominant for the achondroplasia allele is fatal. Embryos that are homozygous dominant frequently miscarry or are stillborn. Heterozygous individuals survive to birth but are affected with achondroplasia throughout their entire life. Imagine you are a genetic counselor advising a young couple who want to have a child together. Both the woman and the man have achondroplasia. Complete the Punnett square to see their possibilities for allele inheritance. (Note: The father's genotype has already been filled in for you. Complete the mother's genotype at the top of the square, and remember that dominant alleles are always listed first.) Then use the Punnett square to answer the question. What is the probability that a child they have will not have achondroplasia? 25% 100% 75%
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Both parents have achondroplasia, so they are both heterozygous for the allele. The father's genotype is Aa. Therefore, the mother's genotype must also be Aa. Show moreā¦
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Achondroplasia is a common cause of dwarfism in humans. All individuals with achondroplasia are thought to be heterozygous at the locus that controls this trait. When two individuals with achondroplasia mate, the offspring occur in a ratio of 2 achondroplasia:1 normal. What is the most likely explanation for these observations? The allele that causes achondroplasia is a dominant lethal allele. Achondroplasia is codominant to the normal condition. The allele that causes achondroplasia is a recessive lethal allele. The allele that causes achondroplasia is a late-onset lethal allele. Achondroplasia is incompletely dominant to the normal condition.
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Achondroplasia is an autosomal dominant disorder characterized by disproportionate short stature: the legs and arms of people with achondroplasia are short compared with the head and trunk. The disorder is due to a base substitution in the gene, located on the short arm of chromosome $4,$ that encodes fibroblast growth factor receptor 3 (FGFR3). Although achondroplasia is clearly inherited as an autosomal dominant trait, more than $80 \%$ of the people who have achondroplasia are born to parents with normal stature. This high percentage indicates that most cases are caused by newly arising mutations; these cases (not inherited from an affected parent are referred to as sporadic. Studies have demonstrated that sporadic cases of achondroplasia are almost always caused by mutations inherited from the father (paternal mutations). In addition, the occurrence of achondroplasia is higher among the children of older fathers; approximately $50 \%$ of children with achondroplasia are born to fathers older than 35 years of age. There is no association with maternal age. The mutation rate for achondroplasia (about $4 \times 10^{-5}$ mutations per gamete) is high compared with those for other genetic disorders. Explain why most spontaneous mutations for achondroplasia are paternal in origin and why the occurrence of achondroplasia is higher among older fathers. (FIGURE CAN'T COPY)
Genetics Many genetic traits are controlled by two genes, one dominant and one recessive. In Gregor Mendel's origi- nal experiments with peas the genes controlling the height of the plant are denoted by T (tall) and t(short). The gene T is dominant, so a plant with the geno type (genetic makeup) TT or Tt is tall, whereas one with genotype tt is short. By a statistical analysis of the offspring in his experiments, Mendel concluded that offspring inherit one gene from each parent and that each possible combination of the two genes is equally likely. If each parent has the genotype Tt, then the following chart gives the possible genotypes of the offspring: Find the probability that a given offspring of these parents will be (a) tall $\quad$ (b) short
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