Ap Bio Chapter 11 Vocab

Concept 11.1: Mendel used the scientific approach to identify two laws of inheritance

• Gregor Mendel formulated a theory of inheritance based on experiments with garden peas, proposing that parents pass on to their offspring discrete genes that retain their identity through generations. This theory includes two “laws.”

• The law of segregation states that genes have alternative forms, or alleles. In a diploid organism, the two alleles of a gene segregate (separate) during meiosis and gamete formation; each sperm or egg carries only one allele of each pair. This law explains the 3:1 ratio of F₂ phenotypes observed when monohybrids self-pollinate. Each organism inherits one allele for each gene from each parent. In heterozygotes, the two alleles are different: expression of the dominant allele masks the phenotypic effect of the recessive allele. Homozygotes have identical alleles of a given gene and are true-breeding.

• The law of independent assortment states that the pair of alleles for a given gene segregates into gametes independently of the pair of alleles for any other gene. In a cross between dihybrids (individuals heterozygous for two genes), the offspring have four phenotypes in a 9:3:3:1 ratio.

Concept 11.2: Probability laws govern Mendelian inheritance

• The multiplication rule states that the probability of two or more events occurring together is equal to the product of the individual probabilities of the independent single events. The addition rule states that the probability of an event that can occur in two or more independent, mutually exclusive ways is the sum of the individual probabilities.

• The rules of probability can be used to solve complex genetics problems. A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously. In calculating the chances of the various offspring genotypes from such crosses, each character is first considered separately and then the individual probabilities are multiplied.

Concept 11.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics

• Extensions of Mendelian genetics for a single gene:

• Extensions of Mendelian genetics for two or more genes:

• The expression of a genotype can be affected by environmental influences. Polygenic characters that are also influenced by the environment are called multifactorial characters.

• An organism’s overall phenotype reflects its overall genotype and unique environmental history. Even in more complex inheritance patterns, Mendel’s fundamental laws still apply.

Concept 11.4: Many human traits follow Mendelian patterns of inheritance

• Analysis of family pedigrees can be used to deduce the possible genotypes of individuals and make predictions about future offspring. Such predictions are statistical probabilities rather than certainties.

• Many genetic disorders are inherited as simple recessive traits. Most affected (homozygous recessive) individuals are children of phenotypically normal, heterozygous carriers.

• The sickle-cell allele has probably persisted for evolutionary reasons: Heterozygotes have an advantage because one copy of the sickle-cell allele reduces both the frequency and severity of malaria attacks.

• Lethal dominant alleles are eliminated from the population if affected people die before reproducing. Nonlethal dominant alleles and lethal alleles that are expressed relatively late in life are inherited in a Mendelian way.

• Many human diseases are multifactorial—that is, they have both genetic and environmental components and do not follow simple Mendelian inheritance patterns.

• Using family histories, genetic counselors help couples determine the probability of their children having genetic disorders.