Modification of Mendelian Ratios

• The effect of alleles on a given phenotype.

• Gene Interaction-control of a single phenotype by more than one set of genes.

• X-linkage

Alleles

• Alternate forms of the same gene.

• Wild type(normal)-occurs most frequently in a population.

-usually dominant

• Derived from mutations-deleterious

-non-deleterious

Symbols For Alleles

• Recessive Allele- d (dwarf)

• Dominant Allele- D (tall)

• D. melanogaster

-wild-type trait= superscript +

- a slash (/) between letters=same locus (location)

Factors that modify the mendelian patterns of inheritance

(1) Incomplete (partial) Dominance

• Neither allele is dominant

• F₁ Heterozygotes= Intermediate (blended) phenotype (Red X White =Pink)

• Phenotypic ratio = Genotypic ratio (1:2:1)

• Tay-Sach’s disease-Enzymatic disorder (hexosaminidase)

-Homozygous recessive = die

-Heterozygotes- 50% of enzyme activity.

(2) Codominance

• Both alleles of a single gene are expressed.

• MN blood group

-Antigens (glycoproteins) on the surface of RBCs.

-Two forms = M and N

• An Individual may exhibit on or both.

(3) Multiple Alleles

• Three or more alleles of the same gene.

(e.g.) (a) The ABO blood group

(b) The Rh antigens

(a) The ABO blood group

• One of four possible phenotypes resulting from 2 antigens (A and B).

• Determined by mixing blood sample with antiserum containing A or B antibodies.

^• Allele Designation- I^A, I^B I^O

• Testing compatibility of blood transfusions

• Determining parentage

• A individual= I^AI^A or I^AI^O

^• B individual= I^BI^B or I^BI^O

• AB individual = I^AI^B

• O individual= I^OI^O

• Expression of ABO influenced by gene at the secretor locus.

• Se/Se or Se/se= secretors

• se/se= non-secretors

(b) Rh Antigens

• Erythroblastosis fetalis (form of anemia)

• Rh-positive fetus/ Rh-negative mother

Rh Antigens may result from

(1) Multiple alleles at a single locus, or

(2) Three or more linked genes, each with 2 alleles

(4) Lethal Alleles

• Recessive lethal allele-Homozygous (lethal)

-Heterozygous (mutant phenotype)

• Dominant lethal allele

-One copy of allele results in death -Huntington’s Disease

-late onset

(5) Combinations of Two Gene Pairs

• 9:3:3:1 ratio modified

• Two modes of inheritance- 3:6:3:1:2:1

• E.g. = 2 individuals heterozygous for albinism and both have AB blood type.

(6) Gene Interaction

• Epistasis- Several genes control one phenotype

• Epistatic

• Hypostatic

• A ratio expressed in 16 parts (9:3:4) when studying inheritance of a single character = two gene pairs are interacting.

• Examples of epistasis

• A genotype mask /suppresses expression of an allele.

• A dominant allele at one locus mask expression of alleles at a second locus.

• The homozygous condition of either recessive allele mask the expression of the dominant allele at the other locus.

• New phenotype in the F2 generation.

X-linkage

• Transmission of genes located on the x-chromosome.

• Crisscross pattern of inheritance.

• Males are hemizygous for all alleles on the x-chromosome.

Extranuclear Inheritance(Non-Mendelian Inheritance)

Maternal Inheritance-DNA from the mitochondria (chloroplast) determines offspring phenotype.

Maternal Effect- Phenotypic effects on the offspring produced by factors transmitted

through the egg cytoplasm.

-Patterns (traits) established during early development.