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Chapter 4: Problem 5

The creeper gene in chickens causes short and stunted legs (creeper condition) in the heterozygous state (Cc) and lethality in the homozygous state (CC). The genotype \(c c\) produces normal chickens. What ratio is obtained when creeper chickens are Interbred? Is the \(C\) allele behaving dominantly or recessively in causing lethality?

Short Answer

Expert verified
Also, does the C allele behave dominantly or recessively in causing lethality? Answer: When creeper chickens (Cc) are interbred, the ratio of genotypes of the offspring is 2 creeper (Cc) : 1 normal (cc). The C allele behaves dominantly in causing lethality.

Step by step solution

01

Set up the Punnett Square

A Punnett square is a grid that allows us to visualize the possible offspring genotypes resulting from a given mating event. Since both parents are heterozygous Creeper chickens (Cc), we will represent one parent's alleles on the top row of the grid and the other parent's alleles on the left column. ``` C c C [ ] [ ] c [ ] [ ] ```
02

Fill in the Punnett Square

Now, fill in the empty squares by combining the corresponding alleles from the parents: ``` C c C [CC] [Cc] c [Cc] [cc] ```
03

Determine Offspring Ratios

From the filled-in Punnett Square, we can see the following genotypes: - 1 CC (lethal) - 2 Cc (creeper) - 1 cc (normal) Remember that CC is lethal, so we will not include it in our ratio. Therefore, the ratio of genotypes obtained when creeper chickens are interbred is: 2 creeper (Cc) : 1 normal (cc)
04

Determine Dominant or Recessive Behavior for Lethality

Lethality occurs in the homozygous state (CC). Since the presence of two dominant C alleles leads to lethality and having only one C allele (heterozygous state) does not lead to lethality, the C allele behaves dominantly in causing lethality. In summary, the C allele is dominant in both causing creeper condition in heterozygous state and causing lethality in the homozygous state.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Punnett Square
A Punnett Square is a simple, visual tool used in genetics to predict the genotypes of offspring resulting from a particular genetic cross. It helps us understand how genetic traits might be passed from parents to their offspring. To use a Punnett Square, draw a grid where one parent's alleles are placed along the top and the other parent's alleles are placed along the side. During breeding, the alleles from each parent combine to fill in the squares of the grid, representing potential offspring genotypes.

In our example of the creeper chickens, each parent chicken has a heterozygous genotype, represented as Cc. As shown in the Punnett Square:
  • C from one parent can combine with C from the other parent to give CC.
  • C from one parent can combine with c from the other parent to give Cc.
  • c from one parent can combine with C from the other to give another Cc.
  • c from one parent can combine with c from the other parent to give cc.
This way, the Punnett Square helps us see all possible combinations and predict outcomes such as creeper, normal, or lethal genotypes.
Dominant Allele
In genetics, alleles determine traits and characteristics. A dominant allele demonstrates its trait by overpowering a recessive allele, which means only one dominant allele is needed to express its trait. This is important when determining phenotypes from genotypes.

In the creeper chicken example, the allele C is dominant. When present, even in a single copy (as in Cc), it causes the trait鈥攃reeper condition鈥攖o show in the chicken. Conversely, for traits only to show with recessive alleles, both alleles must be recessive (cc). For lethality due to C, it requires two dominant alleles (CC) because this lethal trait is dominant, causing death in homozygous state.
Lethal Gene
A lethal gene can cause death when present in particular genetic compositions, known as lethal genotypes. These genes can be challenging to work with in genetic breeding as they must be carefully managed to avoid unwanted mortality in offspring.

In our creeper chicken scenario, the gene causing the creeper trait C is lethal in its homozygous form (CC). This means that when both parents pass the allele C to their offspring, the resulting CC genotype is lethal, and the chick will not survive. This fact must always be considered in breeding strategies, as it significantly impacts expected offspring ratios. For creeper chickens, the lethal gene results in a practical ratio of 2 (Cc):1 (cc) as the CC does not survive, highlighting the importance of understanding gene interactions in genetics.

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Most popular questions from this chapter

Pigment in the mouse is produced only when the \(C\) allele is pres- ent. Individuals of the ce genotype have no color, If color is present, it may be determined by the \(A\) and \(a\) alleles. AA or Aa results in agouti color, whereas aa results in black coats. (a) What \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) genotypic and phenotypic ratios are obtained from a cross between \(A A C C\) and aace mice? (b) In the three crosses shown here between agouti females whose genotypes were unknown and males of the aacc genotype, what are the genotypes of the female parents for each of the following phenotypic ratios? (1) 8 agouti (2) 9 agouti (3) 4 agouti 8 colorless 10 black \(\quad 5\) black 10 colorless

Hemophilia is an X-linked recessive mutation In humans that causes delayed blood clotting. What kinds of \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) offspring would be expected from matings between (a) a hemophilic female and a normal male, and (b) a hemophilic male and a normal female? Compare these results to those that would be obtained if the hemophilic gene was autosomal.

Duchenne muscular dystrophy (DMD), marked by muscular degeneration, results from an \(\mathrm{X}\) - linked recessive gene. Thus, a female who is heterozygous for this gene and does not have the disease can be a carrier. What kind of offspring can you expect from a DMD-affected male and a carrier female? Can there be a carrier male?

Morbid obesity in humans is an autosomal recessive disorder. Rett syndrome is a neurological disorder that is marked by the X.linked dominant allele, If two non-obese parents with Rett syndrome produce an obese, non-Rett son, what is the probability that their next child will be a female who is obese and also has Rett syndrome?

In goats, development of the beard is due to a recessive gene. The following cross involving true-breeding eoats was made and carried to the \(\mathrm{F}_{2}\) generation: \(\mathrm{P}_{1}=\) bearded female \(\times\) beardless male \(\mathrm{F}_{1}:\) all bearded males and beardless females \\[ \mathbf{F}_{1} \times \mathbf{F}_{1} \rightarrow\left\\{\begin{array}{l} 1 / 8 \text { beardless males } \\ 3 / 8 \text { bearded males } \\ 3 / 8 \text { beardless females } \\ 1 / 8 \text { bearded females } \end{array}\right. \\] Offer an explanation for the inheritance and expression of this trait, diagramming the cross. Propose one or more crosses to test your hypothesis.
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