Chapter 9 Patterns of Inheritance

I. Mendel's Laws

1. Heredity is the transmission of traits from one generation to the next.  Genetics is the scientific study of heredity.

2. In the 1860s, Gregor Mendel deduced genetic principles by breeding garden peas.

3. Mendel's law of segregation

The first Mendel's law is the law of segregation describing the segregation of the two copies of alleles of a particular gene into the gametes.

(1)  A monohybrid cross is a mating of individuals differing at one genetic locus.

(2)  P (parental) generation is a monohybrid cross between a true-breeding pea plant with purple flowers (i.e., genotype is PP) and a true-breeding pea plant with white flowers (pp).

(3)  F₁ (filial) generation has all purple flowers (Pp).

(4)  F₂ generation by mating the F₁ with each other has 3/4 purple flowers and 1/4 white flowers in phenotype.  Phenotypic ratio is 3 purple : 1 white, while genotypic ratio is 1 PP : 2 Pp : 1 pp.

(5)  Mendel developed 4 hypotheses:

1) The units that determine heritable traits are alleles (genes).

2) Homozygote has two identical alleles for a given gene, for example, PP or pp in genotype.  Heterozygote has two different alleles for a given gene, for example, Pp in genotype.

3) (Complete) dominant allele represents the allele that determines the phenotype with respect to a particular gene in heterozygotes (the law of dominance; dominant principle).  Recessive allele represents the allele that has no noticeable effect on the phenotype with respect to a particular gene in heterozygotes.

4) The law of segregation states that the two alleles in a pair segregate into different gametes during meiosis.  Therefore, a gamete (sperm or egg) carries only one allele for each inherited character.  (Recall from Chapter 8 that diploid organisms containing pairs of homologous chromosomes produce haploid gametes containing a single set of chromosomes by meiosis I.)

(6)  The cross tracking one character, for example, flower color, can be diagramed by a Punnett square.

(7)  The Punnett square is a diagram that is used to predict an outcome of a particular cross or breeding experiment.  It is named after Reginald C. Punnett, a British geneticist, who devised the approach.

4. Mendel's law of independent assortment

The second Mendel's law is the law of independent assortment describing the independent assortment of alleles of different genes from each other during the formation of gametes.

(1)  A dihybrid cross is a mating of individuals differing at two genetic loci.

(2)  Mendel extended P generation from a monohybrid cross to a dihybrid cross between round-yellow seeds (RRYY) and wrinkled green seeds (rryy).  The allele for round (R) shape of seed is dominant to the allele for wrinkled (r) shape, and the allele for yellow (Y) seed color is dominant to the allele for green (y) color.

(3)  F₁ generation has all round-yellow seeds (RrYy).

(4)  F₂ generation by mating the F₁ with each other has 9/16 round-yellow seeds, 3/16 round-green seeds, 3/16 wrinkled-yellow seeds and 1/16 wrinkled-green seeds in phenotype.  Phenotypic ratio is 9:3:3:1, while genotypic ratio is 1 RRYY : 1 RRyy : 2 RRYy : 2 RrYY : 4 RrYy : 2 Rryy : 2 rrYy : 1 rrYY : 1 rryy.

(5)  A dihybrid cross is equivalent to two monohybrid crosses.  For example, the F₂ ratio between round and wrinkled seeds for a dihybrid cross is 3 : 1 (9+3 : 3+1), which is the F₂ ratio for a monohybrid cross.

(6)  The law of independent assortment states that each pair of alleles assorts independently of the other pairs of alleles during gamete formation.  The inheritance of one character has no effect on the inheritance of another.

5. The family pedigree is a family genetic tree representing the occurrence of heritable traits in parents and offspring across a number of generations.  A pedigree can be used to determine genotypes of matings that have already occurred.

II. Variations on Mendel's Laws (Non-Mendelian Inheritance)

1. Violation of the first Mendel's law (the law of segregation): Recall from Chapter 8 that chromosomal nondisjunction fails to separate a pair of homologous chromosomes.

2. Violation of the second Mendel's law (the law of independent assortment): Linked genes on the same chromosome tend to be inherited together that will be discussed later.

3. Violation of dominant principle:

    (1) Incomplete dominance (semi-dominance, intermediate inheritance) results in intermediate phenotypes. For instance, when red snapdragons are crossed with white snapdragons, all the hybrids have the third phenotype pink flowers.

   (2). Gene polymorphism: many genes have more than two alleles in the population. For example, the ABO blood type in humans involves 3 codominant alleles of a single gene (co-dominance).

   (3). Gene pleiotropy: a single gene may affect many phenotypic characters, for example, sickle-cell anemia.

   (4.) Polygenic inheritance: a single character may be influenced by many genes, for instance, skin color.

   (5) Irregular dominance (reduced penetrance): some individuals will not express the trait even though they carry the defective allele with variable penetrance, such as polydactyly and familial breast cancer due to mutations in the BRCA1 gene.

   (6) Delayed dominance: the penetrance of the defective allele is proportional to age, such as 1%, 38%, and 94% at the ages 20, 40, and 60, respectively. Huntington disease (HD) is an autosomal dominant condition caused by CAG-triplet repeat expansions on the human chromosome 4p16.

III. The Chromosomal Basis of Inheritance

1. The chromosome theory of inheritance accounts for Mendel's laws.

2. The chromosome theory of inheritance was proposed by Theodor Boveri and Walter Sutton in the early 1900s. Genes located on the chromosomes are the units of heredity.

3. Genes on the same chromosome (linked genes) tend to be inherited together discovered by Reginald Punnett and William Bateson in 1905. Linked genes generally do not follow Mendel's law of independent assortment (non-Mendelian inheritance), for instance, flower color (Pp) and pollen shape (Ll) of sweet pea.

4. Chromosomal crossover (crossing over; chiasma (pl. chiasmata) X; fragment exchange; homologous recombination) produces new combinations of alleles.

(1)  In 1900s, Thomas Hunt Morgan studied chromosomal crossover in fruit fly Drosophila melanogaster.

(2)  A heterozygous gray fly with long wings (GgLl) is mated with a black fly with vestigial wings (ggll). If the genes were not linked, independent assortment would produce offspring in a phenotypic ratio of 1:1:1:1 (1/4 gray body with long wings, 1/4 black body with vestigial wings, 1/4 gray body with vestigial wings, and 1/4 black body with long wings).

(3)  Because these genes are linked, Morgan obtained the results that most offspring had parental phenotypes (41.5% gray body with long wings and 41.5% black body with vestigial wings), but 17% (recombinant frequency = number of recombinant/F2 total numbers = (206 + 185)/2300 = 17%) were recombinants (8.5% gray body with vestigial wings and 8.5% black body with long wings).

5. A linkage map is a diagram of relative locations of linked genes on the same chromosome. One % of recombinant frequency between two linked genes represents 1 centimorgan (cM) or 1 map unit (MU). The recombinant frequency is proportional to the distance between two linked genes.

6. A three-point test cross is a technique used in genetic mapping when considering the inheritance of linked three alleles. It can be used to order three loci on a chromosome, and map the distance between these loci in centimorgans (cM).

IV. Sex Chromosomes and Sex-linked Genes

1. Sex chromosomes determine the genetic basis of sex in many species. The X-Y system is a sex-determining system in humans, for instance, XY chromosomes in males and XX chromosomes in females.

2. Y chromosome contains the SRY (sex-determining region of Y) gene plays a crucial role in testis (pl. testes) development.

3. Grasshoppers and some other insects have an X-O (absence) system with X in males and XX in females. Birds, fishes, and butterflies have a Z-W system with ZZ in males and ZW in females. Ants and bees have females developed from fertilized eggs (diploid; 2n = 32) and males developed from unfertilized eggs (haploid; n = 16).

4. A gene located on either sex chromosome is called a sex-linked gene. There are 3 different sex-linked patterns of inheritance: X-linked dominant and recessive inheritances, and Y-linked inheritance (holandric inheritancce).

5. Human sex-linked recessive disorders located on the X chromosome (from his mother) affect mostly males. For example, hemophilia, Duchenne muscular dystrophy, and red-green color blindness are X-linked recessive disorders.

6. Human vitamin D resistant rickets (VDRR; rachitis in Greek) is an X-linked dominant inheritance.

7. Human hypertrichosis of external auditory meatus (HEAM) is a Y-linked inheritance (holandric inheritance).