Principles of Inheritance and Variation Test - 85

Human skin colour is an example of polygenic (multiple gene) inheritances. Assume that three "dominant" capital letter genes (A, B and C) control dark pigmentation because more melanin is produced. The "recessive"alleles of these three genes (a, b & c) control light pigmentation because lower amounts of melanin are produced. The words dominant and recessive are placed in quotation marks because these pairs of alleles are not truly dominant and recessive as in some of the garden pea traits that Gregor Mendel studied. A genotype with all "dominant" capital genes (AABBCC) has the maximum amount of melanin and very dark skin. A genotype with all "recessive" small case genes (aabbcc) has the lowest amount of melanin and very light skin. Each "dominant" capital gene produces one unit of color so that a wide range of intermediate skin colours are produced, depending on the number of "dominant" capital genes in the genotype. For example, a genotype with three "dominant" capital genes and three small case "recessive" genes (AaBbCc) has a medium amount of melanin and an intermediate skin color. This latter genotype would be characteristic of a mulatto.
In the following cross between two mulatto genotypes (AaBbCc x AaBbCc), each parent produces eight different types of gametes and these gametes combine with each other in 64 different ways resulting in a total of seven skin colours. The skin colours can be represented by the number of capital letters, ranging from zero (no capital letters) to six (all capital letters).
 Note: Skin colour may involve at least four pairs of alleles with nine (or more) shades of skin colour.
The cross can be shown with the binomial expansion (a + b)6 where the letter a = number of capital letters and the letter b = number of small case letters. Each term in the expression represents the number of offspring with a specific skin colour phenotype based on the number of capital letters in the genotype. For example, 20 offspring have three capital letters in their genotype and have a skin colour that is intermediate between very dark with all caps (AABBCC) and very light with no caps (aabbcc).
(a + b)6 =   a6 +  6 a5b +  15 a4b2 +  20 a3b3 +  15 a2b4 +  6 ab5 +  b6
Multiple gene (polygenic) inheritance explains many plant and animal traits where there is a wide variation between extreme phenotypes, with most individuals having intermediate phenotypes. In polygenic inheritance the "dominant" capital genes are additive, each capital gene adding one unit of color to the genotype. With more capital genes, the phenotype (appearance) gets darker. The garden peas studied by Gregor Mendel involved pairs of alleles with only three possible genotypes and two phenotypes per trait. For example, the gene for round pea (R) is dominant over the gene for wrinkled pea (r) and only three genotypes are possible: RR, Rr and rr. These three genotypes produce only two phenotypes: Round (RR and Rr) and wrinkled (rr). There are no intermediate traits between round and wrinkled. If all human characteristics were controlled by simple pairs of dominant and recessive alleles like the one Mendel studied, we would have tall and short people with no intermediates. Polygenic inheritance is yet another exception to Mendel's genetic ratios.

Incomplete dominance is the condition when none of factors of a gene is dominant, the phenotype of heterozygous dominant individual is blend of dominant and recessive traits. Here, monohybrid cross between two pure varieties gives 1 : 2 : 1 phenotypic ratio in F$$_2$$ generation which is 3 : 1 in otherwise dominant traits.  Hence, option A is incorrect. The phenotypic ratio of F$$_2$$ generation of a dihybrid cross is 9 : 3 : 3 : 1 provided that the alleles of both follow dominant-recessive relationship. Hence, option B is incorrect. A trait governed by more than one gene where dominant allele of each gene express only a part of trait and the full trait is expressed only in presence of dominant alleles of all multiple genes, is called polygenic inheritance. The phenotype of the organism depends upon number of dominant alleles. A cross between two individuals which are heterozygous for two alleles produce phenotypic ratio of 1 : 4 : 6 : 4 : 1. Hence, option C is incorrect. A cross between two individuals which are heterozygous for three alleles will produce phenotypic ratio of 1 (all dominant alleles): 6 (atleast one recessive allele) : 15 (atleast two recessive alleles) : 20 (three recessive alleles) : 15 (four recessive alleles) : 6 (five recessive alleles) : 1 (all recessive alleles). Hence, the correct option is D. 

The Boveri-Sutton chromosome theory (also known as the chromosome theory of inheritance or the Sutton-Boveri theory) is a fundamental unifying theory of genetics that identifies chromosomes as the carriers of genetic material. It correctly explains the mechanism underlying the laws of Mendelian inheritance by identifying chromosomes with the paired factors (particles) required by Mendel's laws. It also states that chromosomes are linear structures with genes located at specific sites called loci along with them. It states simply that chromosomes, which are seen in all dividing cells and pass from one generation to the next, are the basis for all genetic inheritance.

Germinal mutation is the alteration in the genetic constitution of the reproductive cells, occur in the cell divisions that result in sperm and eggs. Germinal mutations can be caused by radiation or chemical mutagens and may affect a single gene or an entire chromosome. A germinal mutation affects the progeny of the individual in whose reproductive cells the mutation arose and subsequent generations of that progeny. Unlike somatic mutations, which occur in the body cells and are not passed on to later generations, germinal mutations are important sources of genetic variation in natural populations that lead to evolutionary change through natural selection.

Polygenic inheritance occurs when one characteristic is controlled by two or more genes. Often, the genes are large in quantity but small in effect. For example, height, skin colour, eye colour and weight etc.
Polygenic Inheritance was first studied by Kolreuter (1760), while studying tobacco plants and referred to as the 'Father of Polygenic Inheritance'.

Melandruim album (2n = 24), a dioecious species with heteromorphic sex chromosomes (XY in males and XX in females).

In the given case, inheritance is controlled by three genes. So, it is a case of polygenic inheritance. In this type of inheritance, the dominant alleles have cumulative effect with each dominate allele expressing a part of the trait, the full being shown only when all the dominant alleles are present.

Multiple alleles is a type of non-Mendelian inheritance pattern that involves more than just the typical two alleles that usually code for a certain characteristic in a species. With multiple alleles, that means there is more than two phenotypes available depending on the dominant or recessive alleles that are available in the trait and the dominance pattern the individual alleles follow when combined together.
Gregor Mendel only studied traits in his pea plants that showed simple or complete dominance and had only two alleles that could contribute to any one trait the plant showed. It wasn't until later that it was discovered that some traits can have more than two alleles that code for their phenotypes. This allowed many more phenotypes to be visible for any given trait while still following Mendel's Laws of Inheritance.
Most of the time, when multiple alleles come into play for a trait, there is a mix of types of dominance patterns that occur. Sometimes, one of the alleles is completely recessive to the others and will be masked by any of those that are dominant to it. Other alleles may be co-dominant together and show their traits equally in the phenotype of the individual. There are also some cases where some alleles exhibit incomplete dominance when put together in the genotype. An individual with this type of inheritance connected to its multiple alleles will show a blended phenotype that mixes both of the alleles' traits together.
Pleiotropy occurs when one gene influences multiple, seemingly unrelated phenotypic traits, an example being phenylketonuria, which is a human disease that affects multiple systems but is caused by one gene defect.

The cross of white eyed female with red eyed male gives red eyed females and white eyed males. Rarely this cross may give all white eyed females and red eyed males. This results from non-separation of two homologous X chromosomes during anaphase I of meiosis. Both X chromosomes go together to same pole resulting in formation of one gamete with XX and other with no X-chromosome. Fertilization of XX gamete with Y-sperm forms XXY (white eyed female) and fertilization of gamete with no X-chromosome to X-sperm forms red eyed males (X). This phenomenon is known as non-disjunction of two X chromosomes. 

A) Correct option (D)
B) Explanation of the correct options

• According to pedigree, offspring of normal parents show the trait which means that the trait is recessively present in the normal parents; the trait is recessive.
• In a first filial generation, both son and daughter show the trait.
• As we know that a sex-linked recessive trait can be expressed in either sex, not in both; the X-linked trait is expressed in male progeny while the female progeny serves as a carrier and the Y-linked trait is expressed in sons only.
• The presence of trait in both son and daughter in $$F_1$$ generation confirms that it is an autosomal trait. 
• The second parent generation must have a carrier parent and a recessive homozygous parent to pass it to 1/3rd of progeny.
• Phenylketonuria is an autosomal recessive trait.