Biology: Genetics study guide: Chapter 14.1: Human Chromosomes
What patterns of inheritance do human traits follow?
1. Many human traits follow a pattern of simple dominance (Ex. A gene known as MCIR helps determine skin and hair color). 2. The alleles for many human genes display codominant inheritance. (Ex. ABO blood group, determined by a gene with three alleles: I^A, I^B, and i. Alleles I^A and I^B are codominant. They produce molecules known as antigens on the surface of red blood cells). 3. Because the X and Y chromosomes determine sex, the genes located on them show a pattern of inheritance called sex-linkage. Genes on the Y chromosome are found only in males and are passed directly from father to son. Genes located on the X chromosome are found in both sexes, but the fact that men have just one X chromosome leads to some interesting consequences. (Ex. humans have three genes responsible for color vision, all located on the X chromosome. In males, a defective allele for any of these genes results in colorblindness, an inability to distinguish certain colors. The most common form, red-green colorblindness, occurs in about 1 in 12 males. Among females, however, colorblindness affects only about 1 in 200. Why is there such a difference? In order for a recessive allele, like colorblindness, to be expressed in females, it must be present in two copies, one on each of the X chromosomes. This means that the recessive phenotype of a sex-linked genetic disorder tends to be much more common among males than among females.
If a woman with type O blood and a man with type AB blood have children, what are the children's possible genotypes?
50% type A and 50% type B
Define genome.
A genome is the full set of genetic information that an organism carries in its DNA.
How can you use karyotypes to identify a species?
A karyotype cell shows the complete set of chromosomes in a cell, and are displayed in order by size. The number and size of chromosomes varies from one species to the next.
What is a karyotype?
A karyotype is a picture from photographs that shows the complete diploid set of chromosomes grouped together in pairs, arranged in order of decreasing size.
Define pedigree.
A pedigree shows the presence or absence of a trait according to the relationships between parents, siblings, and offspring and can be used for any species, not just humans. It is used to analyze the pattern of inheritance followed by a particular trait in a chart that shows the relationship within the family. We can infer the genotypes of family members. (Ex. See attached image of how one human trait- a white lock of hair just above the forehead-passes through three generations of a family. The allele for the white forelock trait is dominant. At the top of the chart is the grandfather who had the white forelock trait. Two of his three children inherited the trait. Three grandchildren have the trait, but two do not.
What does a pedigree show?
A pedigree shows the presence or absence of a trait according to the relationships between parents, siblings, and offspring.
Describe what makes up a human karyotype.
A typical human cell has 23 pairs of chromosomes. These chromosomes in the image have been cut out of a photograph and arranged to form a karyotype.
Explain how sex-linked traits are inherited?
Because the X and Y chromosomes determine sex, the genes located on them show a pattern of inheritance called sex-linkage.
Looking at attached image, what does this Punnett square suggest about the sex ratio of the human population?
Males and females are born in a roughly 50:50 ratio.
What determines whether a person is male or female?
Sex chromosomes are one of two chromosomes that determine an individual's sex; females have two X chromosomes; males have one X and one Y chromosome.
Codominant and multiple alleles
The alleles for many human genes display codominant inheritance. (ex. ABO blood group, determined by a gene with three alleles: I^A, I^B, and i. Alleles I^A and I^B are codominant. They produce molecules known as antigens on the surface of red blood cells. See attached image of individuals with alleles I^A and I^B produces both A and B antigens, making them blood type AB. The i allele is recessive. Individuals with alleles I^AI^A or I^Ai produce only the A antigen, making them blood type A. Those with I^BI^Bor I^B i alleles are type B. Those homozygous for the i allele (ii) produce no antigen and are said to have blood type O. If a patient has AB-negative blood, it means the individual has I^A and I^B alleles from the ABO gene and two Rh^- alleles from the Rh gene.
Why would the Y chromosome be unlikely to contain any of the genes that are absolutely necessary for survival?
The human Y chromosome is much smaller than the X chromosome and contains only about 140 genes, most of which are associated with male sex determination and sperm development.
How can pedigrees be used to analyze human inheritance?
The information gained from pedigree analysis makes it possible to determine the nature of genes and alleles associated with inherited human traits.
Define autosomal chromosomes.
To distinguish them from the sex chromosomes, the remaining 44 human chromosomes are known as autosomal chromosomes or autosomes. To quickly summarize the total number of chromosomes present in a human cell- both autosomes and sex chromosomes- biologists write 46, XX for females and 46 XY for males.
True or False: Based on a pedigree, you can often determine if an allele for a trait is dominant ore recessive, autosomal, or sex-linked.
True
True or False: More than 1200 genes are found on the X chromosome.
True
True or False: Some of the recessive alleles of the MC1R gene cause red hair. An individual with red hair usually has two of these recessive alleles.
True
True or False: The human Y chromosome is much smaller than the X chromosome and contains only about 140 genes, most of which are associated with male sex determination and sperm development.
True
Define sex chromosomes.
Two of the 46 chromosomes in the human genome are known as sex chromosomes because they determine an individual's sex. Male: 1 (X) and 1 (Y) Female: 2 (X)
