Genetics - Problem Drill 06: Pedigree and Sex Determination

Transcription

1 Genetics - Problem Drill 06: Pedigree and Sex Determination No. 1 of The following is a pedigree of a human trait. Determine which trait this is. (A) Y-linked Trait (B) X-linked Dominant Trait (C) X-linked Recessive Trait (D) Not linked to sex chromosomes (E) None of the above A. Incorrect! This trait seems to affect males only, but females can be carriers, which mean that it is not Y-linked. An X-linked dominant trait would affect both male and females, and the father gives the trait to daughter. Here, the father in the second generation did not give the phenotype to a daughter, suggesting it is not a dominant trait. C. Correct! The female carrier gives the trait to all her sons, which is a typical characteristic for X-linked recessive traits. And the male from the second generation gives the carrier allele to his daughter, confirming the recessive trait and X-linkage. This trait only affected males, so it is sex-linked. There is a correct choice above. Sex-linked traits are transmitted just like all other gene-determined traits. The only difference is that the genes are located on one of the sex chromosomes and, therefore, it is linked to sex. (C)X-linked Recessive Trait

2 No. 2 of Why would you predict that half of human babies born will be males and half will be females? (A) Because of the segregation of the X and Y chromosomes during male meiosis. (B) Because of the segregation of X chromosomes during female meiosis. (C) Because all eggs contain an X chromosome. (D) Because, on average, half of all eggs produce females. (E) Because of random X-inactivation. A. Correct! Sex is determined by the sex chromosome in the sperm that fertilizes the female egg. Half of the sperm have an X-chromosome and half have a Y-chromosome. During female meiosis, all resulting eggs contain one X chromosome; therefore, the eggs cannot determine the baby s sex. During female meiosis, all resulting eggs contain one X chromosome; therefore, the eggs cannot determine the baby s sex. Eggs do not produce any offspring if they are not fertilized. X-inactivation leads to some genes on the X chromosome to lose their function during development, but it is not a sex-determination mechanism. The key to solve this problem is to fully understand how eggs and sperm are produced via meiosis. (A)Because of the segregation of the X and Y chromosomes during male meiosis.

3 No. 3 of Klinefelter s syndrome (XXY) is an example of chromosomal aneuploidy that can be readily determined by: (A) Behavior analysis (B) Somatic cell genetics (C) Karyotyping (D) Biochemical analysis (E) Pedigree analysis A. Incorrect! Behavior analysis can give hints but can never be certain about a syndrome. Somatic cell genetics can give information about somatic cells, not sex-linked traits. C. Correct! Klinefelter s syndrome is a trisomy with an extra X chromosome; karyotyping is the direct detecting method to see it. Biochemical analysis can be used, but it would require a large amount of work; it is rarely used in practice. The syndrome may be caused by X chromosome nondisjunction, which cannot be deduced from pedigree analysis. Karyotyping is the most useful tool in diagnosis of chromosomal abnormality. (C)Karyotyping

4 No. 4 of A human geneticist determined the pedigree shown in the diagram, with filled symbols showing the affected individuals. How is this pattern of inheritance described? (A) Autosomal dominant (B) Autosomal recessive (C) Sex-linked recessive (D) Sex-linked dominant (E) Not enough information to determine A. Correct! A genetic trait that is passed from generation to generation to generation, from both fathers to daughters and mothers to daughters, is typically autosomal dominant. Autosomal recessive may skip generations, but this trait did not skip a generation. The trait is passed from father to sons and daughters; therefore, it cannot be sexlinked. The trait is passed from father to sons and daughter; therefore, it cannot be sexlinked. There is sufficient information given to pick an answer above. One of the characteristics of a sex-linked trait is that it follows certain patterns that are linked to sex; if it is not, it is likely an autosomal trait. (A)Autosomal dominant

5 No. 5 of A female Drosophila of unknown genotype was crossed with a white-eyed male fly, of genotype X W Y (w = white eye allele is recessive, w+= red-eye allele is dominant.) Half of the male and half of the female offspring were red-eyed, and half of the male and half of the female offspring were white-eyed. What was the genotype of the female fly? (A) X W+ Y (B) X W+ X W+ (C) X W X W (D) X W Y (E) X W+ X W A. Incorrect! Drosophila sex determination system is XY; all females have XX chromosome types. If the female had this genotype, all of her progeny would be red-eyed because all the female gametes will carry a X W+ chromosome, which is dominant to X W. This female s offspring would be all white eyes because all the eggs will carry X W chromosome, and the one from the male gamete is also X W chromosome. Drosophila sex determination system is XY; all females have XX chromosome types. E. Correct! The female must be heterozygous because she can transmit either the red eye or white eye allele to her sons and daughters. In this example, the female must be heterozygous because she can transmit either the red eye or white eye allele to her sons and daughters. (E)X W+ X W

6 No. 6 of Which of the following statements about sex determination is correct? (A) A monoecious plant has both male and female reproductive structures. (B) A monoecious plant has either a male or female reproductive structure; the environment dictates which one is present at any one time. (C) Humans have a total of 44 chromosomes, of which 2 are sex chromosomes. (D) Humans have a total of 46 chromosomes, of which 4 are sex chromosomes. (E) Females have a single X chromosome. A. Correct! A monoecious plant has both male and female reproductive structures. A monoecious plant has both male and female reproductive structures. Humans have a total of 46 chromosomes, of which 2 are sex chromosomes. Humans have a total of 46 chromosomes, of which 2 are sex chromosomes. Females have 2 X chromosomes. Sex determination can be genetically or environmentally determined in some plants; an example would be holly. Diecious organisms have either male or female reproductive structures. In humans, sex or gender determination is done by the X and Y chromosomes. XX is a female and XY is a male. (A) A monoecious plant has both male and female reproductive structures.

7 No. 7 of What is meant by haplodiploid? (A) Sex determination where an unfertilized egg develops into a female of the species. (B) Sex determination where a fertilized egg develops into a male of the species. (C) A 2N individual will be a male. (D) A 1N individual will be initially male and then develop into a female. (E) Gender is determined by the number of chromosomes. A. Incorrect! Sex determination where an unfertilized egg develops into a female of the species. In this type of sex determination, females are formed from fertilized eggs. A 2N individual will be a female, as they have the most or double the number of chromosomes. The initial number of chromosomes determines the sex of the species; this does not change. E. Correct! Gender is determined by the number of chromosomes. In some species, gender is determined by chromosome number. In bees, a 2N individual will be female and a 1N individual will be a male. Males develop from single unfertilized eggs and have haploid number of chromosomes. This is called haplodiploidy. (E)Gender is determined by the number of chromosomes.

8 No. 8 of Which species gender is determined by the environment? (A) Some species of turtles and frogs (B) Humans (C) Mammals (D) Cats and dogs (E) None of the above A. Correct! There are some species of turtles and frogs whose gender is determined by the temperature of the environment. Humans have sex chromosomes and the presence of a Y chromosome dictates a male. Most mammals use fertilization of an egg and chromosomal sex determination. Cats and dogs, like most mammals, use chromosomal sex determination. Choice A is the correct answer. Gender, in some species, may be determined by the environment. For instance, temperature has this effect on some turtles and frogs. (A) Some species of turtles and frogs

9 No. 9 of Nondisjunction. (A) Is an abnormal event that leads to aneuploidy. (B) Is a normal event that results in a normal chromosome distribution. (C) Can occur only during meiosis I. (D) Occurs during meiosis II and is called secondary nondisjunction. (E) Occurs during meiosis I and is called primary nondisjunction. A. Correct! Nondisjunction is an abnormal event that leads to aneuploidy. Nondisjunction is an abnormal event that leads to aneuploidy. Nondisjunction can occur in either meiosis I or meiosis II. Secondary nondisjunction is when nondisjunction occurs in an existing aneuploidy cell. Primary nondisjunction is when nondisjunction occurs in a cell with a normal set of chromosomes. For example: When white-eyed females are crossed with red-eyed males, the expected offspring should all be females with red eyes, males with white eyes. Calvin Bridges observed that about 1:2000 females had white eyes and that about 1:2000 males had red eyes. This means that, during meiosis, there is nondisjunction of the X chromosome. (A) Is an abnormal event that leads to aneuploidy.

10 No. 10 of Which of the following statements about ploidy is true? (A) Aneuploidy is the addition of a complete chromosome in all cases. (B) Aneuploidy is the deletion of a complete chromosome in all cases. (C) Monoploidy is the loss of an entire set of chromosomes. (D) Trisomy 2N+1 designates an individual in which a single chromosome is lost. (E) Trisomy 2N+1 designates an individual who has lost 2 copies of a chromosome. A. Incorrect! Aneuploidy is the addition or deletion of all or part of a chromosome. Aneuploidy is the addition or deletion of all or part of a chromosome. C. Correct! Monoploidy is the loss of an entire set of chromosomes. Trisomy 2N+1 designates an individual who has gained an extra copy of a chromosome. Trisomy 2N+1 designates an individual who has gained an extra copy of a chromosome. Aneuploidy: Addition or deletion of all or part of a chromosome. Monoploidy: The loss of an entire set of chromosomes. Euploidy: An entire set of chromosome is duplicated once or several times. Nullisomy: 2N-2 the loss of both pairs of homologous chromosomes; individuals are nullisomics. Monosomy, 2N-1 the loss of a single chromosome; individuals are monosomics. Trisomy, 2N+1 the gain of an extra copy of a chromosome; individuals are trisomics. Tetrasomic, 2N+2 the gain of an extra pair of homologous chromosomes; individuals are tetrasomics. (C)Monoploidy is the loss of an entire set of chromosomes.