what is genetics? the scientific study of heredity
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Gregor Mendel
Born in 1822 in Czechoslovakia.Became a monk at a monastery in 1843.Taught biology and had interests in statistics.Also studied at the University of Vienna
Mendel continued
After returning to the monastery he continued to teach and worked in the garden.
Between 1856 and 1863 he grew and tested over 28,000 pea plants
Mendel’s experiments
The first thing Mendel did was create a “pure” generation or true-breeding generation.He made sure that certain pea plants were only able to self pollinate, eliminating unwanted traits.He did this by cutting away the stamen, or male part of each flower
Genes and dominance
Trait : a characteristicMendel studied seven of these traitsAfter Mendel ensured that his true-breeding generation was pure, he then crossed plants showing contrasting traits.
He called the offspring the F1 generation or first filial.
What will happen when pure yellow peas are crossed with
pure green peas?All of the offspring were yellow.
Hybrids = the offspring of crosses between parents with contrasting traits
What did Mendel conclude?
Inheritance is determined by factors passed on from one generation to another.
Mendel knew nothing about chromosomes, genes, or DNA. Why?
These terms hadn’t yet been defined.
What were Mendel’s “factors”
The ‘factors” that Mendel mentioned were the genes.
Each gene has different forms called alleles
Mendel’s second principle stated that some alleles are dominant and some are recessive.
Mendel’s second cross
He allowed the F1 generation to self-pollinate thus producing the F2 generation.
Did the recessive allele completely disappear?
What happened when he crossed two yellow pea hybrid (F1) plants?
Results:
¾ of the peas were yellow, ¼ of the peas were green.During the formation of the sex cells or gametes, the alleles separated or segregated to different gametes. (pollen and egg)
Probability
The likelihood of a particular event occurring. Chance
Can be expressed as a fraction or a percent.
Example: coin flip.
Punnett Square
Developed by Reginald Punnett.
A diagram used to show the probability or chances of a certain trait being passed from one generation to another.
Reading Punnett squares
Gametes are placed above and to the left of the square
Offspring are placed in the square.
Capital letters (Y) represent dominant alleles.
Lower case letters (y) represent recessive alleles.
Homozygous = when an organism possesses two identical alleles. ex.YY or yy
Heterozygous = when an organism possesses different alleles. ex.Yy
Phenotype vs genotype
Genotype The genetic makeup Symbolized with
letters Tt or TT
Phenotype
Physical appearance of the organism
Expression of the trait
Short, tall, yellow, smooth, etc.
Probability and statistics
No one event has a greater chance of occurring than another.
You cannot predict the precise outcome of an individual event.
The more trials performed, the closer the actual results to the expected outcomes.
Independent Assortment
The two factor cross. Example: color and shape of peas.
F1 cross to produce the F2 generation
Ex: RRYY x rryy
Round yellow mated with wrinkled green• Offspring would all be hybrid for both
traits (RrYy)
Mendel’s death
Mendel published his paper on heredity in 1866.
The scientific community saw little if any importance in his work.
Mendel died in 1884 with no recognition for his contributions to genetics.
Some exceptions to Mendel’s principles:
Some alleles are neither dominant nor recessive.
Many traits are controlled by more than one gene (polygenic traits)
Incomplete dominance
A situation in which neither allele is dominant.
When both alleles are present a “new” phenotype appears that is a blend of each allele.
Alleles will be represented by capital letters only.
Japanese four-o-clock flowers
Red flower plant genotype = RR
White flower plant genotype = WW
Pink flower plant genotype = RW
What happens when a red flower is crossed with a
white flower?
According to Mendel either some white and some red or all offspring either red or white.All are pink
Codominance
When two alleles both appear in the phenotype.
Usually signified using superscripts.
example: color of hair coat in cattle.
crcr = red hairs
cwcw = white hairs
crcw = roan coat (mixture of both colors)
Multiple allele inheritance
When two or more alleles contribute to the phenotype.
Human blood types: A,B,O and AB
A and B are codominant to each other.
Both A and B are dominant over O.
Sample Problem:
A man with type AB blood marries a woman with type B blood whose father has type O blood. What are the chances that they have a child with type A blood? Type AB?
Rediscovery of Mendel’s work
Around the turn of the century (early 1900’s) many scientists “rediscovered” Mendel’s work
1908 – Garrod
1902 – Sutton
1910 – Morgan
Thomas Hunt Morgan
1866-1945
Born in Kentucky, professor of Biology at Columbia U.
Worked with fruit flies (drosophila)
Nobel Prize in Medicine (1933)
Why the Fruit Fly?
1. Can work with large numbers of flies easily
2. Produce many offspring
3. Short reproductive cycle
4. Only four pairs of chromosomes
Meiosis
A method of cell division similar to mitosis.
2 main differences:
1. There are two divisions to produce 4 daughter cells
2. The cells produce contain ½ the chromosomes as the original cell
Chromosome number
All cells of an organism contain a specific number of chromosomes.
Most cells are diploid (2n) meaning they have two copies of each chromosome
Events of meiosis I
During prophase I, each chromosome pairs with its homologous chromosome to form a tetrad
Crossing-over
Crossing-over: an exchange of genetic material between sister chromatids
Results in greater variation
Meiosis II
Neither cell replicates its chromosomes.
Each cell splits (similar to mitosis)
Produces four daughter cells.
Animation
Net result:
Spermatogensis
4 mature sperm
Each sperm has exactly half the number of chromosomes as the father.
Oogensis
1 mature ova or egg.
Each egg has exactly half the number of chromosomes as the mother.
Gene Linkage
Are genes “linked” to each other on chromosomes?
Morgan found that many genes are linked together.
It was determined that chromosomes, not genes, assort independently during meiosis.
Gene Maps
First developed by Sturtevant in 1911.The farther apart two genes are, the more likely they will be separated in meiosis.