Ch. 2 - Transmission Genetics

Sanders - Genetic Analysis: An Integrated Approach 3rd Edition

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Sanders 3rd Edition

Ch. 2 - Transmission Genetics

Problem 44a

Chapter 2, Problem 44a

Situs inversus is a congenital condition in which the major visceral organs are reversed from their normal positions. Investigations into the genetics of this abnormality revealed that individuals with at least one dominant allele (SI) of an autosomal gene are normal but, surprisingly, of individuals that are homozygous for a recessive allele (si), 1/2 are situs inversus and 1/2 are normal.

What genotypes and phenotypes are expected in progeny from a cross of two si si individuals?

Verified step by step guidance

Step 1: Begin by identifying the genotypes of the parents in the cross. Both parents are homozygous recessive (si si), as stated in the problem.

Step 2: Determine the possible gametes produced by each parent. Since both parents are si si, the only gamete they can produce is si.

Step 3: Perform a Punnett square analysis to predict the genotypes of the progeny. Cross the si gametes from each parent to determine the offspring's genotype.

Step 4: Analyze the genotypes of the progeny. All offspring will inherit one si allele from each parent, resulting in a homozygous recessive genotype (si si).

Step 5: Interpret the phenotypes of the progeny based on the problem's description. Among si si individuals, 1/2 are expected to exhibit situs inversus, while the other 1/2 are expected to be normal.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Genotype and Phenotype

Genotype refers to the genetic constitution of an individual, specifically the alleles they possess for a particular gene. Phenotype, on the other hand, is the observable physical or biochemical characteristics of an organism, which result from the interaction of its genotype with the environment. In the context of the question, understanding the difference between these two terms is crucial for predicting the traits of the progeny.

Dominant and Recessive Alleles

Alleles are different forms of a gene that can exist at a specific locus on a chromosome. A dominant allele, represented as 'SI' in this case, will express its trait even if only one copy is present, while a recessive allele, 'si', requires two copies to express its trait. This concept is essential for understanding how traits are inherited and how they manifest in the offspring of the given cross.

Punnett Square

A Punnett square is a diagram used to predict the genotypes and phenotypes of offspring from a genetic cross. By organizing the possible gametes from each parent, it allows for a visual representation of the inheritance patterns. In this scenario, using a Punnett square to analyze the cross between two 'si si' individuals will help determine the expected genotypes and phenotypes of their progeny.

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Textbook Question

Humans vary in many ways from one another. Among many minor phenotypic differences are the following five independently assorting traits that (sort of) have a dominant and a recessive phenotype: (1) forearm hair (alleles F and f )—the presence of hair on the forearm is dominant to the absence of hair on the forearm; (2) earlobe form (alleles E and e)—unattached earlobes are dominant to attached earlobes; (3) widow's peak (alleles W and w)—a distinct 'V' shape to the hairline at the top of the forehead is dominant to a straight hairline; (4) hitchhiker's thumb (alleles H and h)—the ability to bend the thumb back beyond vertical is dominant and the inability to do so is recessive; and (5) freckling (alleles D and d)—the appearance of freckles is dominant to the absence of freckles. In reality, the genetics of these traits are more complicated than single gene variation, but assume for the purposes of this problem that the patterns in families match those of other single-gene variants.

If a couple with the genotypes Ff Ee Ww Hh Dd and Ff Ee Ww Hh Dd have children, what is the chance the children will inherit the following characteristics?

the genotype Ff EE Ww hh dd

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Textbook Question

In chickens, the presence of feathers on the legs is due to a dominant allele (F), and the absence of leg feathers is due to a recessive allele (f). The comb on the top of the head can be either pea-shaped, a phenotype that is controlled by a dominant allele (P), or a single comb controlled by a recessive allele (p). The two genes assort independently. Assume that a pure-breeding rooster that has feathered legs and a single comb is crossed with a pure-breeding hen that has no leg feathers and a pea-shaped comb. The F₁ are crossed to produce the F₂. Among the resulting F₂, however, only birds with a single comb and feathered legs are allowed to mate. These chickens mate at random to produce F₃ progeny. What are the expected genotypic and phenotypic ratios among the resulting F₃ progeny?

A pure-breeding fruit fly with the recessive mutation cut wing, caused by the homozygous cc genotype, is crossed to a pure-breeding fly with normal wings, genotype CC. Their F₁ progeny all have normal wings. F₁ flies are crossed, and the F₂ progeny have a 3:1 ratio of normal wing to cut wing. One male F₂fly with normal wings is selected at random and mated to an F₂ female with normal wings. Using all possible genotypes of the F₂ flies selected for this cross, list all possible crosses between the two flies involved in this mating, and determine the probability of each possible outcome.

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Textbook Question

Situs inversus is a congenital condition in which the major visceral organs are reversed from their normal positions. Investigations into the genetics of this abnormality revealed that individuals with at least one dominant allele (SI) of an autosomal gene are normal but, surprisingly, of individuals that are homozygous for a recessive allele (si), 1/2 are situs inversus and 1/2 are normal.

What genotypes and phenotypes are expected in progeny from a cross of two SI si individuals?

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Textbook Question

Domestic dogs evolved from ancestral gray wolves. Wolves have coats of short, straight hair and lack 'furnishings,' a growth pattern marked by eyebrows and a mustache found in some domestic dogs. In domestic dogs, coat variation is controlled by allelic variation in three genes. Recessive mutant alleles in the FGF5 gene result in long hair, while dogs carrying the dominant ancestral allele have short hair. Likewise, recessive mutant alleles in the KRT71 gene result in curly hair, whereas dogs with an ancestral dominant allele have straight hair. Dominant mutant alleles in the RSPO2 gene cause the presence of furnishings, while dogs homozygous for the ancestral recessive allele have no furnishings. A pure-breeding curly- and long-haired poodle with furnishings was crossed to a pure-breeding short- and straight-haired border collie lacking furnishings

What are the genotypes and phenotypes of the puppies?

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Textbook Question

If dogs of the F₁ generation are interbred, what proportions of genotypes and phenotypes are expected in the F₂?

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