Topic -Mendelian Genetics

Unit 3 - Genetics

OverviewThe passing of traits

from parents to offspring (from one generation to the next) is called inheritance or heredity

Mendel’s ExperimentGregor Mendel is known as

the father of genetics Solved the mystery of

inheritance by studying pea plants (which can consistently produce offspring with only one form of a trait, like flower color or seed shape; they also self-fertilize and grow quickly)

Mendel decided to find out what would happen if he crossed two plants that had 2 different alleles (different forms of the same trait - seed shape)

Mendel’s Experiment cont.Mendel first crossed plants

with round seeds with plants with wrinkled seeds

The offspring from this cross are known as the first filial (F1) generation

The first generation (F1)came up with all round seeds

He took those offspring and allowed them to self-fertilize. The second generation (F2) came up with mostly round seeds, but there were a few wrinkled seeds too (it was an almost 3:1 ratio) – so…

Why would some traits always appear, while some wouldn’t?

Genes and AllelesMendel realized that

each parent donates one set of genes (a piece of your DNA with instructions for a specific trait) to the offspring

The two forms of the gene are known as allelesEx – eye color; you have

a gene for eye color, but you have alleles for blue, brown, or green

Dominant vs. RecessiveThe trait that always

appeared was called the dominant trait (the stronger trait – only needed one allele to be expressed)

The other trait that seemed to recede (or fade) into the background was called the recessive trait. It only appears if there is no dominant allele present (so they must occur in pairs)

Incomplete Dominance and CodominanceA hybrid

organism shows a third phenotype --- not the usual "dominant" one & not the "recessive" one ... but a third, different phenotype.

Incomplete Dominance and Codominance With incomplete dominance we get a

blending of the dominant & recessive traits so that the third phenotype is something in the middle Example - red x white = pink.

In COdominance, the "recessive" & "dominant" traits appear together in the phenotype of hybrid organisms

To explain another way: Incomplete dominance is a situation in which one allele is not completely dominant over another, while codominance is a situation in which both alleles of a gene contribute to the appearance of the organism.

Example: sickle-cell disease affects red blood cells and their ability to transport oxygen (instead of round, they are sickle-shaped); because of their shape, they block circulation in small blood vessels. Those who are heterozygous for the trait have both normal and sickle-shaped cells (more common in people of African decent – about 9% of the population) but can lead a normal life since they have enough normal blood cells to transport oxygen.

Homozygous and heterozygousDominant Alleles are

symbolized (written) with capitol letters

Recessive alleles are symbolized with lowercase letters

When you have two dominant or two recessive alleles for the same trait, that is called Homozygous

When you have two different alleles for the same gene (one dominant and one recessive) that is called Heterozygous

Genotype and phenotype…The inherited

combination of alleles is known as the offspring’s genotype

The letters used to represent the allelesExample: pp or PP or Pp

An organism’s physical appearance is known as it’s phenotypeExample: the flower has

purple petals or the person has brown eyes

Mendel’s Laws Law of segregation – During meiosis,

chromosome number is divided in half; the resulting gametes (sex cells) contain only one of the pair of alleles. (The allele pairs separate or segregate during gamete formation, and randomly unite at fertilization).

The two alleles for each trait separate during meiosis at random – like a deck of cards being shuffled (and during fertilization, two alleles for that trait unite randomly)

Law of independent assortment – a random distribution of alleles occurs during gamete formation (genes on separate chromosomes sort independently during meiosis)

Ex – BB, Bb, bb or bB are each equally likely to occur

Punnett SquaresThese are diagrams used

to predict the possible offspring of a cross between two known genotypes (shows the possible combination of alleles from parents)

Male genotypes are on the horizontal side, and females are written on the vertical side, with possible offspring combinations written on the inside

Sex-linked traitsTraits controlled by genes

located on the X chromosome are called sex-linked traits

Because males have only one X chromosome, they are affected by recessive X-linked traits more often than females (because females have another X chromosome that may mask the effect of the other trait)

Example – color blindness. The trait for red-green color blindness is a recessive X-linked trait (about 8% of males in the US)

The chance that an event will occur.“or” means we add“and” means we multiply1/2 x 1/2 = 1/41/2 +1/2 = 2/2 = 1Practice – if you roll a 6-sided die, what is the

probability that you will roll a 1? What is the probability that you will roll a 1 or a 6?

A. 1/6 C. 1/3B. 1/12D. 1Practice - What is the probability that a couple will

have three kids and they will all be girls?C. 3 C. 1/2D. 0 D. 1/8

Probability