Human Genetics: Concepts and Applications (Lewis), 9th Edition

Chapter 4: Single-Gene Inheritance

Chapter Outline

CHAPTER OVERVIEW

1. Modes of inheritance are the rules explaining the common patterns of inheritance.
2. HD and CF demonstrate two important modes of inheritance. HD is autosomal dominant and CF is autosomal recessive.

1. Gregor Mendel carried out "hybridization" experiments with the common garden pea
2. Mendel's hypotheses became the laws of inheritance in modern genetics.
3. Mendel's work re-discovered by Correns, DeVries, and Tschermak in 1900. Period of modern "Mendelian" genetics begins

1. Chose to work with the common garden pea. Because it is capable of self-fertilization, true breeding parental varieties are easy to establish.
2. Monohybrid cross: Hybrid cross involving a single trait with two expressions (i.e. plant height: short vs. tall).
3. Results of a monohybrid cross demonstrates dominant vs. recessive behavior and the law of segregation (i.e. tall pea plants could produce short offspring).
4. Mendel's "elementen" are what we call genes (or alleles).

1. A homozygous individual possesses two identical alleles (i.e. TT or tt). A heterozygous individual possesses two different alleles (i.e. Tt).
2. Phenotype is the outward expression of a trait (i.e. tall vs. short; blue eyes vs. brown eyes).
3. Genotype is the actual genetic makeup of the individual (i.e. TT, Tt, tt).
4. Wild type refers to the most common form. A mutant is a variant that has undergone a mutation (change in the DNA).
5. The physical nature of meiosis (chromosome behavior) explains the law of segregation. The law of segregation states that inherited "characters" (alleles) separate during meiosis, so that each offspring receives one copy of each allele from each parent.
6. Punnett squares are a convenient method for diagramming a genetic cross. Inspection of the square gives you the genotypic and phenotypic results and ratios.
7. The genotypic ratio for a monohybrid cross is 1:2:1, and the phenotypic ratio is 3:1.
8. A test cross reveals the presence of recessive genes in an individual with an unknown genotype by crossing them with an individual homozygous recessive for the genes in question.

1. A Mendelian trait is caused by a single gene.
2. Mendelian human conditions are considered more rare than multifactorial ones.
3. Mendelian inheritance in humans is more complex than in model organisms such as pea plants.

1. The inheritance of human eye color is an example of a single gene trait.

1. Modes of inheritance are the rules explaining the common patterns of inheritance.
2. Genes may be autosomal or on a sex chromosome. Alleles can be dominant or recessive.
3. Autosomal Dominant Inheritance: Autosomal dominant traits do not generally skip generations and can affect both sexes.
4. Autosomal Recessive Inheritance: Autosomal recessive traits can skip generations and can affect both sexes.
5. Blood relatives that have children together have a much higher risk of having a child with a rare recessive disorder.
6. Punnett squares apply Mendel's first law to predict recurrence risks for inherited disorders or traits.
7. A Mendelian trait applies anew to each child.

1. At the biochemical level, recessive disorders often result from alleles that cause a loss of function or loss of a normal protein.
2. Dominant disorders can result from production of an abnormal protein that interferes with the function of a normal protein or imparts a gain of function.

1. Mendel's law of independent assortment considers genes transmitted on different chromosomes.
2. The phenotypic ratio of 9:3:3:1 of a dihybrid cross indicates that a gene on one chromosome does not influence transmission of a gene on a different chromosome.
3. In meiosis, random assortment of maternally and paternally derived chromosomes results in gametes that have different combinations of genes.
4. Punnett squares and probability are used to predict recurrence of more than one trait.

1. Pedigree charts depict family relationships and transmission of inherited traits.
2. Squares represent males and circles represent females.
3. Horizontal lines indicate parents, vertical lines show generations, and elevated horizontal lines depict siblings.
4. Symbols for heterozygotes are half-shaded, and for individuals with a particular phenotype, completely shaded.

1. Pedigrees have been in use since antiquity.
2. They have been used throughout history to show family relationships, particularly royal bloodlines.

1. Pedigrees can reveal mode of inheritance, and can suggest molecular information, carrier status, and input from other genes and the environment.
2. Interpretation of pedigrees can be inconclusive when more than one mode of inheritance can explain the pattern seen.