Lecture 26Genetically Inherited Diseases

The Monk and His Peas

• An Austrian monk, Gregor Mendel, developed the fundamental principles that would become the modern science of genetics.

• Mendel demonstrated that heritable properties are parceled out in discrete units, independently inherited.

• These eventually were termed genes.

Mendel’s Experiments

• Mendel's experimental organism was a common garden pea (Pisum sativum), which has a flower that lends itself to self-pollination.

• The male parts of the flower are termed the anthers. They produce pollen, which contains the male gametes (sperm).

• The female parts of the flower are the stigma, style, and ovary. The egg (female gamete) is produced in the ovary.

• The process of pollination (the transfer of pollen from anther to stigma) occurs prior to the opening of the pea flower.

Mendel’s Experiments

• The pollen grain grows a pollen tube which allows the sperm to travel through the stigma and style, eventually reaching the ovary.

• The ripened ovary wall becomes the fruit (in this case the pea pod).

• Mendel tested all 34 varieties of peas available to him through seed dealers.

• The garden peas were planted and studied for eight years.

• Each character studied had two distinct forms, such as tall or short plant height, or smooth or wrinkled seeds.

Inherited Characteristics

Definitions

• Genotype – the specific allele composition of a cell.

• Phenotype – the detectable manifestation of a specific genotype.

Cross female phenotype: Purple X White :male phenotype

Parent genotype

Purple (P) Purple (P)

White (w) P,w P,w

White (w) P,w P,w

Phenotypically all offspring (F1) produce purple flowers, but each has information for both purple and white pigment production. Therefore, genotypically the offspring are heterozygous. In this case the allele for production of the purple pigment is dominant

Principle of Segregation -Monohybrid Cross

Smooth vs Wrinkled Peas

Cross female phenotype: Purple X Purple :male phenotype

F1 genotype

P w

PP,P

P,w

wP,w

w,w

In looking at these offspring (F2), one can see that phenotypically three of four (3/4) produce purple flowers while one of four (1/4) produce white flowers. We say that such a cross typically produces a 3:1 ratio of dominate to recessive phenotype expression. Genotypically, however, one notes that the ratio is 1(homozygous dominant):2 (heterozygous):1 (homozygous recessive).

F1 x F1 = F2

Dihybrid CrossCross Female = Green pods with Round peas X Yellow pods with Wrinkled peas = Male

Parent genotype Green (G),Round (R)

Green (G), Round (R)

Yellow (y), Wrinkled (w)

Gy,Rw Gy,Rw

Yellow (y),Wrinkled (w)

Gy,Rw Gy,Rw

In looking at the phenotypes of the first generation offspring (F1), we find that all produce green pods and round peas. Genotypically, however, all F1 individuals have both green and yellow pod color and round and wrinkled pea appearance information (heterozygous). Green pod colour and round peas appearance are, therefore, the dominant alleles

F1 X F1 = F2

Parent genotype G,R G, w y,R y,w

G,RGG,RR GG,Rw Gy,RR Gy,Rw

G,wGG,Rw GG,ww Gy,Rw Gy,ww

y,RGy,RR Gy,Rw yy,RR yy,Rw

y,wGy,Rw Gy,ww yy,Rw yy,ww

In looking at the phenotypes, we find nine exhibit the dominant allele for both traits; three that exhibit the dominant green pods but wrinkled (recessive) peas; three that exhibit yellow (recessive) pods but the dominant round peas; one that exhibits the recessive allele for both traits---9:3:3:1. We find a more complex assortment with regard to genotypes

Expression of Alleles

• Dominance refers to the effects of one allele overriding the effects of another allele (of the same gene). For example, A is dominant to a. Dominant traits were defined by Mendel as those which appeared in the F1 generation in crosses between true-breeding strains.

• Recessives were those which "skipped" a generation, being expressed only when the dominant trait is

• Epistasis refers to the effects of one gene overriding the effects of another gene. For example, gene A (really the aa genotype) is epistatic to gene B.

Polygenic Inheritance

• Many traits such as height, shape, weight, color, and metabolic rate are governed by the cumulative effects of many genes.

• Polygenic traits are not expressed as absolute or discrete characters, as was the case with Mendel's pea plant traits.

Multiple Allelic Inheritance

• While many genes have only two possible alleles, and we are most accustomed to looking for inheritance patterns involving only two possible alleles, there are genes for which geneticists have identified more than two possible alleles.

• These traits are said to be multi-allelic. • One of the best documented and more interesting multi-

allelic traits is that of the ABO blood group antigens in humans.

• The phenotype (A, B or O) on the individual's erythrocytes is determined by the presence and identity of the terminal sugar attached to a molecular configuration on the surface of the erythrocyte membrane.