Polygenic Traits

Mendel’s crosses for plant height vs. human height.

The problem with the comparison is that a single gene pair is in control of height in pea plants, whereas several gene pairs determine human heights.

The gene product in plants that controls height is gibberellin

Short plants have a mutation in this gene

Traits that are determined by two or more genes are referred to as polygenic traits

Human height exhibits continuous variation

Pea plant height shows discontinuous variation (tall or short)

Multifactorial Traits

• IQ

Traits that result from the interaction of one or more environmental factors and two or more genes.

Ex

Japanese soldiers during WWII

Old diet was higher in carbs and in comparison to today were on average 3 inches shorter.

Suggesting some kind of environmental influence on a person’s phenotype.

Ex Obesity

Studies have looked at both identical twin and non identical twin and found a strong genetic correlation to obesity.

Pina Indians

Evidence supports the contention that they may possess a “fat gene”

May have been favored in people when food supply was scarce.

But:

1. Hard to conduct environmental studies on identical twins
2. Geographical data may suggest otherwise

OB/OB Mouse Model

• Ob mice were found to have a mutation in the gene producing Leptin
• Leptin serves as an appetite suppressor; in its absence the mice do not receive the signal to stop eating.

• Diabetic mice do not have a Leptin deficiency, rather are unable to translate the signal into the appropriate action.

Down Syndrome

Karyotype

the appearance of the chromosomal makeup of a somatic cell in an individual or species (including the number and arrangement and size and structure of the chromosomes)

2n + 1 (chromosome 21)

          2N + 1                                                         2N – 1

Most fetuses that have an abnormal chromosome number are aborted

3 Ways to get Down Syndrome

1. Trisomy 21  (Sticky Chromosomes)

• Occurs during Meiosis
• Due to non-disjunction at the first/second meiotic division

Note: Euploidy

The normal status of the chromosomes is two pairs of 23 types of autosomal chromosomes and one pair of sex chromosomes.  46XX is female euploidy, 46XY is male euploidy.

v     Where the chromosomes are either

1.     haploid (23)

2.     Diploid (46)

3.     Triploid (67)

v     Anuploid

                                                              i.      Abnormal chromosome number (too few or too many)

1.     Monosomy (2N-1)

2.     Trisomy (2N+1)

Q. Why couldn’t a tetraploid situation occur in humans?  (4N)

Q. What was one possible explanation as to why chromosomes become sticky?

2. Chance events occurring at Mitosis

• These random events have nothing to do with the age of the parent

• Strictly a chance event

• Produces a mosaic effect

• Not all cells have the abnormal chromo number

• 90% normal 10% abnormal

3. Translocation of the chromosomes

Accounts for approx 7% of all D.S. cases

Chances of Downs