Blueprint of Life Topic 13: Co-dominance Biology in Focus, HSC Course Glenda Childrawi, Margaret Robson and Stephanie Hollis

DOT POINT(s) explain the relationship between homozygous and heterozygous genotypes and the resulting phenotypes in examples of co-dominance

Introduction Co-dominance is another example of inheritance that does not show a Mendelian pattern. This is because in genes of some organisms, pairs of alleles do not show dominance of one over the other—that is, they are exceptions to Mendel’s laws of prominence. commons.wikimedia.org

Introduction Co-dominance: In a heterozygote where two different alleles for the same gene are present, both alleles are expressed as separate, unblended phenotypes and so they are termed co-dominant (co = together; that is, both alleles behave as dominant alleles because they are both expressed). www.goldiesroom.org

Examples in Animals Pure-breeding (homozygous) cattle may have a red or white coat colour. Hybrid individuals (heterozygotes), which have one allele for red and one for white coat colour, have a roan appearance—both red and white hairs are present, not in patches but interspersed (that is, both types of hairs are present, indicating that both alleles are expressed, a typical example of co-dominance). www.bio.georgiasouthern.edu

Examples in Animals Another example occurs in Andalusian chickens. If a homozygous black fowl is crossed with a homozygous white fowl, the heterozygous offspring in the F1 generation appear ‘blue’. At first this was thought to be a ‘blending’ of characteristics (a concept called incomplete dominance), but closer examination revealed that the blue Andalusian fowls have both black and white feathers present—a typical example of co-dominance. giselamatthiessen.ifunnyblog.com

Examples in Humans (ABO Blood Groups) Blood grouping is one of the most commonly studied genetic variations in human populations. This is mainly because transfusions of the wrong type of blood can lead to death, but also because the inheritance of blood groups can be used to help determine the parents of a child in paternity suits in courts of law. ken-foundation-awareness2.blogspot.com

Examples in Humans (ABO Blood Groups) Blood cells have proteins on their surfaces called antigens and these play an important role in allowing a person’s own body cells to be recognised by the immune system as ‘self’ (that is, cells belonging to that individual). www.beltina.org

Examples in Humans (ABO Blood Groups) The antigens can be described as the ‘uniform’ that a cell wears so that the immune system will recognise it as ‘belonging’ (not being ‘foreign’). Invading cells have on their surfaces antigens that are different, so the body recognises them as ‘foreign’ or ‘nonself’ and produces antibodies to attack and destroy the foreign invaders. www.nano.org.uk

Examples in Humans (ABO Blood Groups) If the wrong blood group is given in a transfusion, the antigens on the surface of these blood cells are recognised as ‘foreign’ and the recipient’s body will produce antibodies to attack and fight these cells. The clumping reaction can block blood vessels and prove fatal. drerhumu.blogspot.com

Examples in Humans (ABO Blood Groups) There are many systems of antigens present in blood cells, but the two most important systems for transfusions are the ABO system and the Rhesus (positive and negative) system. The inheritance pattern of the ABO system of blood antigens shows co- dominance, whereas that of the Rhesus system shows typical Mendelian dominance.

Examples in Humans (ABO Blood Groups) There are three alleles in the population for this particular gene— alleles A and B code for the presence antigens A and B respectively, but allele O codes for no antigen. As we already know, any one individual will have only have two alleles (one paternal and one maternal) for a particular trait such as ABO blood group. Therefore the possible genotypes are AA, BB, AB, AO, BO and OO. youshotmedown.wordpress.com

Examples in Humans (ABO Blood Groups) If the allele A is present, the blood cells produce a surface antigen known as A. If allele B is present, B antigen is produced. If both A and B alleles are present, blood cells have both A and B surface antigens—both alleles are expressed in each other’s presence. A and B are said to be co-dominant.

Examples in Humans (ABO Blood Groups) If neither antigen is present, the blood cells produce no antigen and are said to be group O. (Note the different use of symbols to represent co-dominant alleles—each allele is represented by a different capital letter, signifying that both alleles are expressed).

Examples in Humans (ABO Blood Groups) Studies of the inheritance of the ABO blood system do not produce Mendel’s ratios. The gene does not follow his law of dominance—there is equal expression of the A and B alleles in the heterozygous form, as these alleles exhibit co-dominance (they are both expressed when present in a heterozygote). www.biologycorner.com

Recognising Genetic Crosses There are three ways to recognise the different types of genetic crosses. These can be used to solve problems involving co- dominance and sex-linkage. Monohybrid crosses Sex-linkage Co-dominance Handout Copies of the following slides for students to glue into their notebook

Recognising Genetic Crosses Monohybrid crosses ■ Only one type of characteristic is involved in the problem (e.g. coat colour) and there are usually only two variations of this (e.g. black coat/white coat). ■ The genotype is written using capital and small (lower case) versions of the same letter and no X or Y chromosomes are shown (capital = dominant; lower case = recessive). ■ The genotype always contains two of the same letter (which may be capital or lower case): —AA or aa = homozygous form —Aa = heterozygous form youshotmedown.wordpress.com

Recognising Genetic Crosses Sex-linkage ■ The sex (male or female) of the parents and the offspring are always mentioned. ■ One sex, usually the male, has only one copy of the gene— that is, if the gene is on the X chromosome, he has only one copy of the gene since the male has one X and one Y chromosome. The female has two X chromosomes and therefore two copies of the gene. youshotmedown.wordpress.com

Recognising Genetic Crosses Sex-linkage ■ The X and Y chromosomes are written into the genotype e.g.: — 𝑋 𝐻 𝑋 𝐻 = normal female; 𝑋 𝐻 Y = normal male — 𝑋 𝐻 𝑋 ℎ = carrier female; 𝑋 𝐻 Y = affected male — 𝑋 ℎ 𝑋 ℎ = affected female. (Note: The X or Y chromosome is indicated by a capital letter and the sexlinked gene is shown as a superscript on the chromosome on which it occurs.) ■ The recessive characteristic appears more frequently in the males than in the females (if it is X-linked), because there is no dominant gene on the Y chromosome to counter its effect.

Recognising Genetic Crosses Co-dominance ■ One type of characteristic (e.g. coat colour in cattle or horses) is involved, but there are usually three variations of this (e.g. red, roan and white coat). ■ The heterozygote exhibits the phenotypes of both homozygotes (e.g. roan = individual white hairs in amongst the red hair)—that is, both phenotypes are ‘equally dominant’ and both are expressed. youshotmedown.wordpress.com

Recognising Genetic Crosses Co-dominance ■ The genotype is written using two different letters, both capital letters: —RR = red coat (homozygous) —WW = white coat (homozygous) — RW = roan coat (heterozygous, co-dominant) ■ No X or Y chromosomes are shown. ■ The genotype contains two letters. These are both capitals and may be two of the same letter or two different letters. youshotmedown.wordpress.com

Activity/Homework -Students to complete DOT Point 3.10 Co-dominance and sex- linkage problems a