Genetics is the science of genes, heredity, and variation in living organisms

GeneticsIntuition will tell the thinking mind where to look next.

- Dr. Jonas Salk (inventor of the Polio vaccine)

Genetics is the science of genes, heredity, and variation in living organisms.

Gregor Johann Mendel (July 20, 1822 – January 6, 1884) was a scientist and Augustinian friar who gained

posthumous fame as the founder of the new science of genetics.

He was a German lad who grew up on the family farm in Austria. He joined the Monastery so he could have a free education. He persisted with his education through two illnesses which caused him to miss over a year of school.

He conducted his study in the monastery's 2 hectares (4.9 acres) experimental garden.

. Mendel demonstrated that the inheritance of certain traits in pea plants follows particular patterns, now referred to as the laws of Mendelian inheritance.

The garden pea plant (Pisum Sativum) is self pollinating. This means that the pollen from a flower fertilizes the egg cells of the same flower. So, the seeds that are produced

have all their traits from one parent..

In pollination the sperm cell in the pollen grain unites with the egg cell in the ovary to eventually become a seed for

a new plant.

This self pollination led to true breeding plants. That is short plants produced seeds that grew into short plants,

etc.

After some initial experimentation, he settled on seven traits to study: seed shape, flower color, seed coat, pod shape, unripe pod color, flower location and plant height

To make crosses between different traits, Mendel would cut off the anthers of a flower and use a paint brush to

add pollen from a plant he wished to cross.

Between 1856 and 1863 he tested some 29,000 pea plants. He discovered that ¼ of the plants were purebred

recessive alleles, ½ were hybrid and ¼ were purebred dominant.

Each original pair of plants is the P (parental) generation.The offspring are called the F1, or first filial generation.The offspring of parents with different traits are called

hybrids.

He felt that traits were passed on from the parents to the young.The factors that determine traits are called genes.Each gene had two different forms or alleles.The gene for height had an allele for Tall (T) and one for short (t)

The allele for tall was dominant over the allele for short.If a plant had both alleles it would be tall but would carry

an allele for short.

Mendal crossed the F1 plants (Tt) with each other to see if the allele for short (t) had disappeared.

He discovered that ¼ of the F2 plants were short

For each F1 plant the alleles segregate when the gametes (eggs and sperm) are produced.

Mendel realized that principles of probability could be used to predict outcomes in the crosses.

Probability concerns the likelihood that an event will occur

The probability that several independent events will occur in a row is the product of their individual

probabilities.So, the probability of flipping two heads in a row is ¼.

What is the probability of a family having 6 boys in a row?

What are the odds that the next child is a boy? Girl?

Here is a family with 7 girls in a row then 6 boys in a row!

What is the probability of that?

Punnett squares are used to determine probabilities in the outcomes of genetic crosses.

The genotype = the alleles that the organism has (i.e. TT or Tt or tt

The Phenotype represents how the organism looks (Tall or short)

The larger the number of flips the more the odds approach 50/50.

The same is true in genetics. The number of boys and girls in the US is about the same but you can still have

families with all boys or all girls.

Homozygous organisms have two of the same allele (HH or hh)

Heterozygous organisms have two different alleles for a trait

Law of independent Assortment.Separate genes for separate traits are passed

independently of one another from parents to offspring

Law of Segregation: When any individual produces gametes, the copies of a gene separate so that each

gamete receives only one copy (allele).

Incomplete dominance occurs when the phenotype of the heterozygous genotype is distinct from and often intermediate to the phenotypes of the homozygous

genotypes.

Codominance is a phenomenon in which a single gene has more than one dominant allele.

If someone is heterozygous for two codominant alleles they will express the phenotypes associated with both.

Multiple Alleles – a situation where there are three or more forms of a gene for a trait. The organism still has

only two but there are three or more alleles in the population

Polygenic traits – Traits that are produced by the interaction of several genes. i.e. skin color.

Fruit Flies (Drosophila melanogaster) are nice to use for research because of their large and few (8) chromosomes, they are easy to care for, they breed quickly and they lay

many eggs.

Physical Appearance of sexes

Some Fruit Fly recessive traits

Meiosis is a type of cell division necessary for sexual reproduction in eukaryotes, such as animals, plants and

fungi.In meiosis, the number of sets of chromosomes is cut in

half from two sets (diploid) to one set (haploid)

Homologous chromosome is a set of one maternal chromosome and one paternal chromosome that pair up

with each other during meiosis. These copies have the same genes in the same locations, or loci, as one another.

Gametes (sex cells) for Fruit flies must contain half the number of chromosomes (haploid or 4) if the adults are to

have 8 chromosomes.

Meiosis is a kind of cell reproduction in which the number of chromosomes is cut in half.

Meiosis occurs in two stages – meiosis I and meiosis IIPrior to Meiosis I each chromosome is replicated.

The replicated chromosomes pair up, maternal with paternal to form a set of four chromatids called a tetrad.

Meiosis I Prophase IChromosomes become visible, crossing over occurs, the nucleolus disappears, the meiotic spindle forms and the

nuclear membrane disappears.

Crossing over in Meiosis I Prophase ICrossing over is the physical exchange of chromosome

parts. This crossing over results in new combinations of alleles

for the offspring

Animated Prophase I of MeiosisProphase I is the longest phase of meiosis, typically

consuming 90% of the time for the two divisions.

Meiosis I: Metaphase IThe pairs of chromosomes (bivalents) become arranged

on the metaphase plate and are attached to the now fully formed meiotic spindle.

Meiosis I: Anaphase IThe two chromosomes in each bivalent separate and

migrate toward opposite poles. Notice that the sister chromatids remain attached!

Meiosis I: Telophase IThe homologous chromosome pairs reach the poles of the cell, nuclear envelopes form around them, and cytokinesis

follows to produce two cells.

Meiosis I: AnimatedThe homologous chromosome pairs reach the poles of the cell, nuclear envelopes form around them, and cytokinesis

follows to produce two cells.

Meiosis II: Prophase IIMeiosis II begins without any replication of the

chromosomes. In prophase II, the nuclear envelope breaks down and the

spindle apparatus forms.

Meiosis II: Metaphase IIThe chromosomes become arranged on the metaphase plate, much as the chromosomes do in mitosis, and are

attached to the now fully formed spindle.

Meiosis II: Anaphase IIThe centromeres separate and the sister

chromatids—now individual chromosomes—move toward the opposite poles of the cell.

Meiosis II: Telophase IIA nuclear envelope forms around each set of

chromosomes and cytokinesis occurs, producing four daughter cells, each with a haploid set of chromosomes.

Meiosis II - Animated

One way meiosis generates genetic variability is through the different ways in which maternal and paternal

chromosomes are combined in the daughter cells.

Another way meiosis generates genetic variability is through the process of crossing-over between maternal

and paternal chromatid pairs during prophase I

Gamete formation in Males (Spermatogenesis)

Gamete formation in Females (Oogenesis)

Mitosis – 2 genetically identical diploid cellsMeiosis – 4 genetically different haploid cells

Thomas Hunt Morgan. Many genes in the fruit fly were inherited together (linkage groups). The four linkage

groups corresponded to the four chromosomes of the flies

If 2 genes are on the same chromosome are they linked forever?

The farther apart two genes are the more likely they are to be separated by a crossover in Meiosis.

The rate of crossing over allowed people to determine locations for many genes on the chromosomes of the fruit

fly.