Genetic Crosses and Phenotypic Ratios

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Questions and Answers

In a scenario where four offspring are produced from a cross, and both parents are heterozygous for a trait (Bb), what explains a deviation from the expected 3:1 phenotypic ratio?

  • The 3:1 ratio is a statistical expectation that is not guaranteed in small sample sizes. (correct)
  • The parents are showing incomplete dominance leading to a different phenotypic ratio.
  • The sample size is large enough to guarantee the expected phenotypic ratio.
  • The alleles are showing complete dominance so the phenotypic ratio will always be 3:1.

What is the primary purpose of performing a test cross?

  • To create new genetic variations within a population.
  • To identify if an organism with a dominant phenotype is homozygous or heterozygous. (correct)
  • To determine the exact number of dominant alleles an organism possesses.
  • To produce offspring with only recessive traits.

If a test cross between an organism with a dominant phenotype and a homozygous recessive organism yields only offspring with the dominant phenotype, what can be inferred about the genotype of the dominant parent?

  • The offspring inherited a new mutation causing them to express the dominant trait.
  • The parent with the dominant phenotype is homozygous dominant. (correct)
  • The parent with the dominant phenotype is heterozygous.
  • The parent with the dominant phenotype carries two different dominant alleles.

In humans, what is the approximate probability of a conceived child being male?

<p>50% (C)</p> Signup and view all the answers

Why is the phenotypic ratio of males to females approximately 1:1 in humans?

<p>Because the probability of a sperm carrying an X or Y chromosome fertilizing the egg is roughly equal. (D)</p> Signup and view all the answers

In X-linked inheritance, why are males more likely to express recessive traits compared to females?

<p>Males only have one X chromosome, so a single recessive allele will be expressed. (C)</p> Signup and view all the answers

In a cross between a healthy man ($X^HY$) and a woman carrier of hemophilia ($X^HX^h$), what is the probability that their son will have hemophilia?

<p>50% (D)</p> Signup and view all the answers

A couple, where the male has hemophilia and the female is homozygous healthy, plan to have a child. What is the probability that their daughter will be a carrier of hemophilia?

<p>100% (C)</p> Signup and view all the answers

A hemophilic man and a homozygous healthy woman have children. What is the probability that their son will inherit hemophilia?

<p>0% (B)</p> Signup and view all the answers

What is the term for the observable characteristics or traits of an organism?

<p>Phenotype (B)</p> Signup and view all the answers

What does a genetic diagram (Punnett square) primarily illustrate?

<p>The probable genotypes and phenotypes resulting from a cross. (A)</p> Signup and view all the answers

What is the term for different versions of a gene at a specific locus?

<p>Allele (C)</p> Signup and view all the answers

What is meant by the term 'homozygous' for a specific trait?

<p>Having two identical alleles for that trait. (C)</p> Signup and view all the answers

Which of the following statements correctly describes the relationship between genotype and phenotype?

<p>Phenotype is the physical expression of a trait, influenced by the genotype and environmental factors. (D)</p> Signup and view all the answers

Flashcards

What is a test cross?

A cross to determine if an individual with a dominant phenotype is homozygous or heterozygous. It involves crossing the individual with a homozygous recessive individual.

Test cross ratio (heterozygous)

If the dominant phenotype individual is heterozygous, a 1:1 phenotypic ratio will be observed in the offspring.

Test cross ratio (homozygous)

If the dominant phenotype individual is homozygous, all offspring will have the dominant phenotype.

Sex chromosome distribution

Normal eggs carry one X chromosome. Sperm carry either an X or Y chromosome.

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Probability of sex

The probability of a zygote being male or female is 0.5 (50%) each.

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Sex inheritance

Males (XY) pass either an X or Y chromosome. Females (XX) always pass an X.

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Sex determination using diagrams

Using genetic diagrams, one can visually trace the sex inheritance from parents to offspring. It illustrates the probabilities of male and female offspring.

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Gamete chromosome

Normal egg carries one X chromosome, sperm carries either X or Y chromosome with equal probability.

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Genetic diagrams (sex-linked)

Illustrates inheritance patterns, especially for sex-linked traits. It tracks how alleles are passed from parents to offspring.

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Hemophilia alleles

An altered allele on the X chromosome causes hemophilia, denoted X^h. A healthy allele is denoted X^H.

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Carrier mother, passing allele

A woman carrier ($X^HX^h$) has a 50% chance of passing the hemophilia allele ($X^h$) to her offspring.

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Hemophilic son chance

There is a 50% chance a son ($X^hY$) of a carrier mother will have hemophilia.

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Healthy Daughters

The probability of a healthy daughter from a healthy man and carrier woman is 1 (she will be either homozygous dominant or a carrier).

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Hemophilic Sons Probability

The probability of a hemophilic son from a healthy man and carrier woman is 0.5 (50%).

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Hemophilic father outcome

All sons of a hemophilic father ($X^hY$) and homozygous healthy mother ($X^HX^H$) will be healthy ($X^HY$). All daughters will be healthy carriers ($X^HX^h$).

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Study Notes

  • More offspring from a single cross leads to the observed phenotypic ratio of the offspring matching the estimated phenotypic ratio.
  • A large number of offspring (e.g., 100) should approach a phenotypic ratio of 3:1 in a cross between F1 offspring.

Test Cross

  • A test cross determines if an individual with a dominant phenotype is homozygous or heterozygous for a trait.
  • It involves crossing the individual with the dominant phenotype with an individual known to be homozygous recessive for the studied trait.
  • A 1:1 phenotypic ratio suggests the individual with the dominant phenotype is heterozygous.
  • All offspring having the dominant phenotype suggests the individual with the dominant phenotype is homozygous.

Sex Determination Using Genetic Diagrams

  • Genetic diagrams can be used to study sex inheritance
  • Normal eggs (female gametes) carry one X chromosome
  • Females have two X chromosomes and no Y chromosomes
  • The probability of an egg carrying an X chromosome is normally 1
  • Sperm (male gametes) carry either a Y chromosome or an X chromosome.
  • Approximately half of sperm carries a Y chromosome and the other half carries an X chromosome
  • The probability that a sperm will contain an X chromosome is 0.5
  • The probability that a sperm will contain a Y chromosome is 0.5.
  • The phenotypic ratio of males to females is 1:1
  • There is a probability of 0.5 (50% chance) that a zygote will be male
  • There is a probability of 0.5 (50% chance) that a zygote will be female

Genetic Diagrams for Sex-Linked Inheritance and Codominance

  • Hemophilia's X-linked inheritance involves the allele for a healthy clotting factor (XH) and the altered allele for hemophilia (Xh).

  • In a cross between a healthy man (XHY) and a healthy woman carrier (XHXh):

    • The probability of the man passing on the healthy X allele (XH) is 0.5, and the probability of passing on the Y chromosome is 0.5.
    • The probability of the woman passing on the healthy allele (XH) is 0.5, and the probability of passing on the altered allele (Xh) is 0.5.

  • The probability of a healthy daughter (either XHXH or XHXh) is 1 where:

    • The healthy daughter with the XHXH genotype is 0.5
    • The healthy carrier of the XHXh genotype is 0.5

  • All female offspring will be healthy; none will have hemophilia, but half will be carriers.

  • The probability of a healthy son (XHY) is 0.5 and of a hemophilic son (XhY) is also 0.5.

  • A son of a woman carrier and a healthy man has a 50% chance of being hemophilic.

  • The phenotypic ratio of healthy to hemophilic males in this cross is 1:1, where there is a 1 in 2 chance of receiving a mutated allele and a 1 in 2 chance of receiving a healthy allele

  • The phenotypic ratio of healthy to hemophilic offspring for all children is 3:1, where there is a 3 in 4 chance the offspring will be healthy and a 1 in 4 chance the offspring will be hemophilic

  • In a cross between a hemophilic man (XhY) and a homozygous healthy woman (XHXH):

    • The woman will pass a healthy allele (XH) to her offspring is 1
    • The man will contribute an X chromosome carrying the altered hemophilic allele, is 0.5, and the probability that he will contribute a Y chromosome, is 0.5

  • The probability that this couple will have a healthy daughter carrier (XHXh) is 1

  • The probability that this couple will give birth to a healthy son (XHY) is 1.

  • None of the sons will receive a mutant hemophilia allele, and all of the daughters will receive only one mutant hemophilia allele

  • None of the children of a hemophilic father and a homozygous healthy woman will exhibit hemophilia.

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