Codominance :  It is a condition in which two alleles of a locus are both fully expressed in the heterozygous form.  A good example of codominance is a person's blood type (MN blood group type).  The M and N are antigens found on the surface of the red blood cell (RBC). Inheritance Beyond Mendel

Inheritance Beyond Mendel - Codominance Red cow White cow Roan Cow

Incomplete Dominance :  In incomplete dominance, the effect of the two alleles is blended.  eg. Inheritance of snapdragon flower colour. Inheritance Beyond Mendel

Incomplete Dominance :  A true-breeding white snapdragon is mated with a true-breeding red snapdragon. Inheritance Beyond Mendel

Incomplete Dominance :  All the offspring in F1 are pink.  What will be the result if two individuals (hybrid heterozygous) from the F1 generation are allowed to self- pollinate? Inheritance Beyond Mendel

RR X WW R W RR RW RW WW Red Pink White

Incomplete Dominance :  A quarter (¼) is white, another quarter (¼) is red and the remaining half (½) are pink.  This type of inheritance, where the heterozygote shows an intermediate phenotype, is known as incomplete dominance. Inheritance Beyond Mendel

Multiple alleles :  Normally, for a given gene, there are two alleles: the dominant and the recessive.  However, with multiple alleles, there can be more than two alternative forms of a gene (alleles) that can occupy the same gene locus on a pair of homologous chromosomes.  However only two of these alleles can occupy a locus in a single diploid organism.  Example : Human ABO blood group - There are 3 different alleles : I A, I B, and I O.

Multiple alleles :  Before donating or receiving blood, the blood type of both donor and recipient must be considered because blood type determines what antibodies are located within the blood.

 Type A blood has type B antibodies.  If type B blood is put into their bodies, their immune system reacts as if it were a foreign invader, the antibodies clump the blood - can cause death.  Type AB blood has no antibodies, any blood can be donated to them. (Universal acceptor)

Lethal genes :  Lucien Cuénot (1905) - Cross breeding between two yellow mice (heterozygous) produces offspring (F1) in the ratio of 2 yellow fur and 1 brown / agouti fur.  This result can only be explained on the basis that yellow is dominant to brown / agouti and that all the yellow fur mice are heterozygous.  Fetal death to the homozygous yellow fur mice.

Recessive lethal genes :  Recessive lethal genes do not actually cause death unless an organism carries two copies of the lethal allele.  So recessive lethal genes cause death only in homozygous condition.  In the case of human, diseases caused by recessive lethal alleles include cystic fibrosis and sickle-cell anemia.

Dominant lethal genes :  Dominant lethal genes cause death in both homozygous and heterozygous conditions.  One example of a disease caused by a dominant lethal allele is Huntington's disease, a neurological disorder in humans, which reduces life expectancy.  It affects muscle coordination and leads to cognitive decline and psychiatric problems.  Since onset / beginning of Huntington's disease is slow, individuals carrying the allele can pass it on to their offspring.

Gene complex (Polygene) :  Most characteristics are determined by the interaction of several genes which form a `gene complex’.  A single characteristic may be controlled by the interaction of two or more genes situated at different loci.  eg. The wide range of skin color (quantitative character) in humans comes about partly because there are three different genes control this trait.  Quantitative character usually indicates that the character is controlled by more than one gene.

A simplified model for polygenic inheritance of skin color

Other quantitative characters in human are : Height and Body weight.

Gene interaction :  Sometimes interaction between two or more genes determines certain characteristic.  This kind of interaction gives more variation on the phenotypes.  The genes that involve are normally situated in different chromosomes.

Gene interaction : Epistasis :  A gene is said to be epistasis when its presence suppresses the effect of a gene at another locus.  Sometimes these genes are called `inhabiting genes’.  Examples : Fur colour in mice is controlled by a pair of genes occupying different loci.

B - Grey colour b - Black colour C - Coloured fur c - Non-coloured fur /albino  The epistasis gene determines the presence of fur colour and has two alleles, coloured (dominant) and albino (recessive).

B - Grey colour b - Black colour C - Coloured fur c - Non-coloured fur /albino  The other gene determines the nature of the colour and its alleles are grey (dominant) and black (recessive).

 The mice may have grey or black fur depending upon their genotypes, but this will only appear if accompanied by the allele for coloured fur.  The albino condition appears in mice that are homozygous recessive for coloured even if the alleles for grey and black fur are present.

Linkage genes  Genes that control two traits are found on the same autosomal chromosome.  When pure breeding pea plants with purple flowers and long pollen grains are crossed with plants with red flowers and short pollen grains, all the F1 plants have purple flowers and long pollen grains.  If two plants from F1 are allowed to self-pollinate, the following results can be obtained from the F2 generation.

PPLLppll X Parents Mendel’s Dihybrid Inheritance  Genes that control two traits are found on different autosomal chromosome.  Key :  P – dominant allele for purple flower  p – recessive allele for red flower  L – dominant allele for long pollen grain  l – recessive allele for short pollen grain

PPLLppll X Parents PLlp Gametes PpLl F1 Genotype – PpLl (heterozygous), all individuals show dominant trait.

PP LL Parents pp ll X P L p l Gametes F1 Pp Ll Linkage Genes Inheritance  Genes that control two traits are found on the same autosomal chromosome. Genotype – PLpl (heterozygous), all individuals show dominant trait.

Testcross – Mendel Inheritance 1 : 2 : 1 Parents PpppLlll Gametes PLPlpLplpl PpppppllPpLlllLl Phenotype ratio 1 : 1 : 1 : 1

Testcross – linkage genes Pp Ll Parents X pp ll Gametes P L p l p l P L p l p l p l Parents phenotype 1 : 1

 When pure breeding pea plants with purple flowers and long pollen grains are crossed with plants with red flowers and short pollen grains, all the F1 plants have purple flowers and long pollen grains.  A testcross is carried out and the following results is obtained. Purple flower / long pollen grain = 252 Red flower / short pollen grain = 238 Purple flower / short pollen grain = 28 Red flower / long pollen = 36

Crossing over  Crossing over is an exchange of corresponding parts of non-sister chromatids between homologous chromosomes.

Crossing over  This occurs during prophase I meiosis and the results are the formation of new allelic combinations in the gametes.  Crossing over results in the recombination of linked genes.

Crossing over of linkage genes Pp Ll Parents vsvs pp ll P L p l Gametes p l P l p L

Determining the distance between linkage genes : Pp Ll Pp Ll  Theoretically if the distance between the two linkage genes is greater, chances of crossing over is higher and vice-versa.  Unit : Linkage Map Unit (LMU) or map unit.

P – dominant allele for purple flower. p – recessive allele for red flower L- dominant allele for long pollen grain l- recessive allele for short pollen grain. After a test cross (PLpl x plpl) Purple flower / long pollen grain = 252 Red flower / short pollen grain = 238 Purple flower / short pollen grain = 28 Red flower / long pollen = 36 Determining the distance between linkage genes :

Purple flower / long pollen grain = 252 Red flower / short pollen grain = 238 Purple flower / short pollen grain = 28 Red flower / long pollen = 36 From the data Distance between allele P and L or p and l = Total no. of individual with recombinant phenotype x 100% = Total no. of individual = (64 / 554) x 100% = map unit.

Sex-linked genes  This mode of inheritance is in contrast to the inheritance of traits on autosomal chromosomes, where both sexes have the same probability of expressing the trait.  Sex-linked genes are located on the sex chromosome (X chromosome, Y chromosome)

 Sex-linked inheritance in Drosophila melanogaster.

 T.H Morgan (1910) discovered a mutant male fly with white eyes instead of red.  Cross the mutant male to a normal red-eyed female fly.

 In Drosophila the locus for eye color is located on the X chromosome.  The allele for red eye color, which is normal in wild flies, is dominant to the mutant allele for white eyes.

Sex-linked genes in human :-  Hemophilia is a bleeding disorder that slows the blood clotting process.  People with this condition experience prolonged bleeding following an injury or surgery.  The genes associated with these conditions are located on the X chromosome thus it affects mostly males, as it is an X chromosome linked condition.