Genetics Part III

There is a short cut way to do dihybrid crosses by working with one gene at time and determining the probability for that gene and do the same for the second gene. Then multiply the two probabilities together. Cross one trait at a time!

Some traits are controlled by more than one gene (multi genetic) Some traits are controlled by more than one gene (multi genetic). These result in a number of different phenotypes. Example, height is controlled by a number of genes. The more genes that are "turned" on, the taller the individual.

Multi genetic trait: *skin color *eye color *height *hair color

Gender is determined by genes found on the sex chromosomes Gender is determined by genes found on the sex chromosomes. It can be different from species to species. The X and Y are different from one another with different genes on them. (Human karyotype on right)

Who determines the sex? Gender determination female male mammal XX XY birds XY XX bees XX X

Mammals- gender determination is caused by sex chromosomes. females XX - homologous chromosomes males XY - not homologous chromosomes

Most populations gender is 50:50 XY * XX ->|XX| half female ->|XY| half male

Humans gender is determined by the male.

There are other genes are on the sex chromosome so therefore they segregate with gender. Sex linked traits are always found on the X chromosomes. Example: color blind sex link is recessive.

Non disjunction- during the process of meiosis is when chromosomes do not separate. You must have 2 copies of every autosome (1-22) 3 copies of any autosome is usually fatal . Exception: Trisomy-21 or Down Syndrome *lowered I.Q. , pudgy, rounded mouth, protruding tongue, heart defects

Trisomy 18 - fatal in infancy

Generalizations 1. Everyone must have at least 1 X to live. 2. If there are more than one X chromosome, the other X chromosome(s) fold up to a barr bodies.

#chromosomes Sex # barr bodies XX female - 1 XY male- 0 OX female- 0 XXY - male 1 XXX- female- 2 XYY- male - 0

3. The presence of a Y chromosome results in maleness no matter how many other X chromosomes are present. 4. The more sex chromosomes a person has, there is an increase height and a decrease in I.Q

Recap: Non disjunction Mutations XO is Turner's syndrome. This results in a female. Tiny in stature, sexually immature without menses or secondary sex characteristics. XXY-Klinefelter's syndrome. This results in a male with small imperfect testes and feminine contours. XYY-Double Y syndrome. This results in a male, extreme height, and lowered I.Q. XXX-Triple X females. Tall in stature and usually sterile.

Recap: Variations in gametes