Genetics: Sex-Linked Genes, Pedigrees, and Genetic Disorders

Posted on May 12, 2024 in Biology

Sex-Linked Genes

Sex-linked genes are located on both the X chromosome and the Y chromosome.

Formation of a Barr Body

The formation of a Barr body inactivates one whole X chromosome in a female cell.

Pedigree in Figure 14–3

Examine the pedigree in Figure 14–3. The allele for the presence of a white forelock is dominant. Therefore, we can tell from the chart that in the couple labeled 2

Trait in Pedigree in Figure 14–5

The trait in pedigree in Figure 14–5 has two alleles: P (dominant) and p (recessive). The black symbols show the dominant phenotype, and the white symbols show the recessive phenotype. What is the genotype of individual number 1?

If the Allele for Having a White Forelock Is Dominant

If the allele for having a white forelock is dominant, family members WITHOUT a white forelock are

Which of the Following Is Caused by a Dominant Allele?

Which of the following is caused by a dominant allele?

Sickle Cell Disease Is Caused by a

Sickle cell disease is caused by a

If Nondisjunction Occurs During Meiosis

If nondisjunction occurs during meiosis,

Which Enzyme(s) in Figure 14–8 Would Be Best for Cutting DNA to Make Fragments with Sticky Ends?

Which enzyme(s) in Figure 14–8 would be best for cutting DNA to make fragments with sticky ends?

What Prevents Insurance Companies from Discriminating Against People Based on Information Derived from Genetic Tests?

What prevents insurance companies from discriminating against people based on information derived from genetic tests?

In humans, the mother’s gamete determines the sex of the offspring. F, father’s gamete

To make a karyotype, biologists take pictures of cells during mitosis, when they are condensed and easier to view. 

TRUE

If the gene for a recessive sex-linked trait was on the X chromosome, the trait would be more common in males than in females.

TRUE

The pedigree chart in Figure 14-10 shows that some people can be carriers of the trait without being afflicted. This means the allele for the trait is dominant.

F, recessive

The pedigree chart in Figure 14–10 shows 4 generations.

F, 2

Down syndrome, Turner’s syndrome, and Klinefelter’s syndrome are all caused by nondisjunction.

TRUE

 When DNA fragments are separated by gel electrophoresis, the longest fragments move fastest.

F, shortest

When sequencing DNA, replication stops when nucleotide bases marked with dye are added to chains of DNA as the DNA is being synthesized.

TRUE

A haplotype is a group of alleles found on the same chromosome that tend to be inherited all together.

TRUE

Information from the Human Genome Project can be used to learn more about human diseases.

TRUE

In humans, sex is determined by the X and Y chromosomes.

If a couple has five boys, the probability that the next child will be a boy is 50%

A boy who is colorblind inherited the disorder from his MOTHER

In humans, there are three different alleles for blood type and two of these alleles are codominant, so there are FOUR possible phenotypes.

A(An) PEDIGREE is a diagram that follows the inheritance of a single gene through several generations of a family.

Examine the pedigree in Figure 14–3. The allele for the presence of a white forelock is dominant. The probability of the couple labeled 2 in the pedigree having a child without a white forelock is 25 percent.

People who have sickle cell disease inherited TWO copies of the sickle cell allele.

Nondisjunction can lead to the disorder called Klinefelter’s syndrome, in which a male has an extra X chromosome.

An advantage of using a restriction enzyme such as EcoR1 is that it creates Sticky Ends that can match to complimentary base pairs.

The law that protects people from being discriminated against because of information learned in genetic tests is called the Genetic Information Nondiscrimination Act

Infer In Figure 14–13, are any of the descendants of individuals 1 and 2 homozygous for free earlobes?

No, none of the descendants are homozygous (FF) for free earlobes.

If human gametes have just 23 chromosomes, why does a human karyotype show 46?

A  sperm that has 23 chromosomes fertilizes an egg that has 23 chromosomes resulting in a 23 pairs of chromosomes (46 total) in the autosomal cells of the individual.

Why are the sex chromosomes considered homologous, even though they vary in size and appearance?

The sex chromosomes are homologous because one sex chromosome is inherited from one parent, and the other is inherited from the other parent.

A man who is not colorblind and a woman who is a carrier of the disorder have a son. What is the probability that their son will be colorblind?

The probability that their son will be colorblind is 50%.

Why are Barr bodies not found in male cells?

Male cells do not have two X chromosomes.

The only people with Rh negative blood are people who have inherited two recessive alleles for this trait. But some people who are colorblind have inherited only one recessive allele for this trait. Explain why these traits are different in terms of how many recessive alleles are needed to produce the recessive phenotype.

The inheritance of Rh blood groups follows a pattern of simple dominance, but the colorblindness is sex-linked. Men who are colorblind receive only one recessive allele, but have no corresponding dominant allele, so they have the recessive phenotype.

Compare and contrast the information you can learn from looking at a pedigree with the information you can learn from a karyotype.

A karyotype gives information about a person’s chromosomes—how many they have and which sex chromosomes they have inherited. A pedigree gives you a person’s phenotype for a specific trait, along with the phenotypes of their relatives.

How is the DNA sequence of the allele that causes cystic fibrosis different from that of the normal allele?

The DNA sequence of the allele that causes cystic fibrosis has a deletion of three bases.

If malaria were eliminated from a certain area, how do you think the frequency of the sickle cell allele in that area would change? Explain.

The frequency of the sickle cell allele would probably decrease because the allele would no longer be beneficial in heterozygous individuals.

How can you tell that the nondisjunction occurred during the formation of the gametes in Figure 14–7?

You can tell that nondisjunction occurred because the gametes have an abnormal number of chromosomes.

In gel electrophoresis, DNA fragments move across a gel. What causes them to move?

When voltage is applied to the gel, one end of the gel becomes positively charged. DNA fragments are negatively charged, so they move toward the positive end.

1. Infer In the human karyotype in Figure 14–11, what term is used to describe the pair of chromosomes in each numbered group?

The chromosomes in each group are called homologous chromosomes.

Draw Conclusions Study the abnormality at position 23 in Figure 14–11. What type of abnormality is this, and how might it have occurred?

The karyotype shows a chromosomal abnormality. Position 23 has 3 chromosomes instead of the usual two. This abnormality was caused by nondisjunction of sex chromosomes during meiosis while a sperm or egg cell was developing in a parent.

Interpret Visuals Which numbered slide in Figure 14–12 shows type B blood?

Slide 2 shows type B blood.

Infer In Figure 14–12, which slide shows the blood of a person who can safely receive any type of blood in a transfusion? Identify the person’s blood type.

Slide 3 shows the blood of a person who can safely receive any type of blood. The person’s blood type is AB.

Interpret Visuals Which slide in Figure 14–12 shows blood that contains no antigens? How do you know?

The blood on slide 4 contains no antigens. There is no clumping in either drop of blood.

Infer In Figure 14–12, what is the genotype or possible genotypes of the person whose blood is shown in slide 1?

The person’s genotype is I^AI^A or I^Ai.

Infer Anti-sera are extracted from blood. Based on what is shown in Figure 14–12, what type of blood does not contain anti-A or anti-B serum? Explain why.

Type AB blood does not contain anti-A or anti-B serum because it contains both antigen A and antigen B. If it did contain either serum, the serum would cause the blood to clump in the person’s body.

Infer Is individual 2 in Figure 14–13 homozygous or heterozygous for free earlobes? Explain.

The individual is heterozygous (Ff), since her daughter has attached earlobes. The daughter inherited one allele for attached earlobes from individual 2 and another from individual 1.

Predict Predict the genotype and phenotype of individual 14 in Figure 14–13.

Individual 14 will be homozygous for the recessive allele (ff) and will have attached earlobes.

How might karyotypes be useful to doctors?

Doctors can use karyotypes to determine the sex of an individual. They can also use karyotypes to determine whether an individual has an abnormal number of chromosomes or noticeable chromosomal mutations, such as large deletions, additions, or translocations.

What can you conclude if you observe a male cat that has both black and orange spots? Explain your answer.

The male cat inherited an extra X chromosome. One of the X chromosomes carries the allele for black spots, while the other carries the allele for orange spots. The two X chromosomes are randomly inactivated in the cat’s cells, producing the orange and black spots.

Explain why the father of a girl who is colorblind must also be colorblind.

Because the allele for colorblindness is recessive and X-linked, the girl must have inherited the alleles for colorblindness on the X chromosomes from both her mother and father. Since the father has a single X chromosome, it must carry the allele for colorblindness, and he must be colorblind.

Why does Huntington disease remain in the human population, even though it is fatal and is caused by a dominant allele?

The symptoms of Huntington disease usually do not appear until a person is middle-aged. Thus, although the allele is dominant, people who have Huntington disease may have one or more children before becoming aware that they are carrying the allele for Huntington disease. The allele is passed on to the children and therefore remains in the population.

Compare and contrast the causes of Turner’s syndrome and Down syndrome.

The two syndromes are similar because both Turner’s syndrome and Down syndrome are the result of nondisjunction, the failure of chromosomes to separate in meiosis. So, in both cases, a fertilized egg has an abnormal number of  chromosomes. The chromosomal problem in each case is different, however. In the case of Turner’s syndrome the fertilized egg has just one sex chromosome. In the case of Down syndrome, the fertilized egg has three copies of chromosome 21.

Compare and contrast Turner’s syndrome and Klinefelter’s syndrome.

Both Turner’s syndrome and Klinefelter’s syndrome result from nondisjunction of the sex chromosomes. A female with Turner’s syndrome inherits only one X chromosome. A male with Klinefelter’s syndrome has at least one extra X chromosome.

 Contrast the role of DNA polymerase in determining a DNA sequence with its role in normal cells.

In DNA sequencing, DNA polymerase replicates small pieces of DNA using normal bases and some bases that are tagged with dye. Each base can be tagged with a different color dye. The dye-tagged bases stop replication, and after running the bases on a gel, scientists can determine the sequence of the DNA by reading the order of the colored bands on the gel. In normal cells, DNA polymerase builds new DNA strands based on the parent strands, but does not use bases that have been tagged with dye.

Why might it be incorrect to assume that if one chromosome is larger than another, the larger chromosome has more genes?

The larger chromosome might be larger because it has longer stretches of repetitive DNA than does the smaller chromosome. Because repetitive DNA does not code for proteins, it does not contain genes. Thus, in this case, the larger chromosome does not have more genes than the smaller chromosome.

Shotgun sequencing was one of the techniques used to sequence the human genome. Below are five DNA fragments¾labeled A, B, C, D, and E, respectively¾that were shotgun sequenced and determined to be part of the same DNA sequence. Notice that the fragments are single stranded. Determine the single-stranded DNA sequence that the fragments below are part of. Explain your reasoning.

Fragment A: GATCTAGGTCATG

Fragment B: ACAG

Fragment C: CAGTCTGATC

Fragment D: AGGTC

Fragment E: CATGCGATC

The DNA sequence is ACAGTCTGATCTAGGTCATGCGATC. The sequence is determined by finding overlapping regions among the five fragments as shown below.