Co-dominant and Incomplete dominance Genetics Monohybrid crosses Co-dominant and Incomplete dominance Sex-linked genes

Definitions Trait – a characteristic that an organism possesses such as height, hair color, eye color Alleles – different genes coding for a trait such as short and tall, blue and brown eyes Homozygous – having two alleles (or genes) that are the same Two genes for brown eyes, two for blue eyes Heterozygous – having two different alleles for a trait Having one gene for brown hair, one for blonde hair Dominant – trait that shows up in a heterozygous Brown eyed individual having genes for blue and brown eyes Written using upper case letters Recessive – allele that is hidden in a heterozygous Gene for blue eyes in a brown-eyed individual Written using lower case letters; use letter of dominant Phenotype – the physical appearance Written as words (tall, short, green-eyed) Genotype – The combination of genes an individual has Written as pairs of letters: TT or Bb or gg

Example of a cross The trait is eye color in bunnies The alleles are for brown and green eyes The mating of true breeding brown-eyed parent #1 with true breeding green-eyed parent #2 All the resulting offspring (F1) will be brown-eyed Dominant = brown eyes Use the letter “B” because brown is dominant Phenotype of first parent is brown-eyed genotype is BB Phenotype of second parent is green eyed genotype is bb (not gg!) Cross is written as BB x bb (for brown-eyed mated with green-eyed) Offsprings’ phenotypes are brown-eyed; their genotypes are Bb

Background Information Each individual is represented by two letters Letters represent the alleles or genes One allele comes from the mother, one from the father Different alleles are found on homologous chromosomes Brown-eyed allele on one chromosome, green-eyed on another Homologous chromosomes from a parent go to separate cells This is called independent assortment Reproductive (sex) cells are haploid Have EITHER one allele or the other 50 : 50 chance as to which allele it will get Homozygous brown-eyed is written as BB Homozygous green-eyed is written as bb Heterozygous brown-eyed is written as Bb

Setting up a monohybrid cross Draw a 2 x 2 box One parent’s diploid alleles go on side of box Put only one allele per row Second parent’s diploid alleles go on top of box Put only one allele per column Inside box, combine letters from top and side Put upper case letters first

Determining Phenotypic Ratios Shown as two numbers, separated by colon 4:0, 2:2, 3:1, 0:4 First number is dominant phenotype Combine homozygous dominant and heterozygous together Second number is recessive phenotype Use only homozygous recessive individuals Having two lower-case letters Include words 4:0 tall : short or 3:1 black : white

Determining genotypic ratios Count homozygous and heterozygous dominants separately Should have 3 numbers separated by colons 1:2:1, 4:0:0 Include genotypic combinations : homozygous dominant, heterozygous, homozygous recessive 1:2:1 GG : Gg : gg

Co-Dominance, Incomplete Dominance and Sex-Linked Genes

Co-dominance Both traits are expressed equally in the offspring Neither trait is dominant over the other Use two different capital letters for these traits Example: blood type A type blood, B type blood If you have A and B type blood, you are AB

Incomplete dominance There is a blending of the two traits in the offspring NO parental trait is present Example: red flower x white flower resulting in pink flowers Use different letters for each trait R for red, W for white RR = red, WW = white, RW = pink

Sex-linked genes X and Y chromosome determines male and female traits as well as other traits Only men inherit the Y chromosome Both men and women inherit the X chromosome Genes on the X chromosome are called sex-linked traits X linked genes are always expressed in males Males have only one copy of genes on the X chromosome 2nd copy is NOT found on the Y chromosome Recessive X-linked genes are not expressed in females Masked by good copy on the 2nd X chromosome Heterozygous female is said to be a carrier for that gene.

Examples of sex-linked genes Hemophilia Cannot control bleeding Lacks protein clotting factor VIII or IX Red-Green color blindness Inability to distinguish between red and green Duchenne Muscular Dystrophy Rapidly progressive muscle weakness Starts in leg and pelvic area Progresses to entire body