Mendelian Genetics

1860's: Gregor Mendel described the fundamental principles of inheritance. Mendel discovered that certain traits show up in offspring plants without any blending of parent characteristics. For example, the pea flowers are either purple or white: intermediate colors do not appear in the offspring of cross-pollinated pea plants.

Terms to Know

• Genetics – study of heredity• Fertilization – process of combining haploid sex cells

(egg and sperm) • true-breeding (pure bred) - organisms that produce

offspring identical to themselves• Trait - specific characteristic EX: eye color• Gene - a functional unit that controls an inherited trait• Allele – alternative form that a single gene may have for

a particular trait. EX: pink or white flowers• Gamete – haploid sex cell as egg or sperm

More Terms to Know• Homozygous – organism with two of the same alleles for

a specific trait. EX: BB• Heterozygous - organism with two different alleles for a

specific trait. EX: Bb• Hybrid – organism that is heterozygous for a specific trait

EX: Bb• Genotype –an organism’s allele pairs. EXs: Bb, BB, bb• Phenotype – observable characteristic that is expressed

as a result of an allele pair. EX: purple flower color• Dominant allele – an expressed allele EX: B• Recessive allele – a hidden or masked allele EX: b

Some Terms Used in Genetics

• Genes• Locus• Diploid cells• alleles• homozygous• heterozygous• dominant allele• recessive allele• phenotype• genotype

Probability

Probability is the likelihood that a specific event will occur or is the likely outcome a given event will occur from random chance.

A Probability may be expressed as a Decimal (0.75), a Percentage (75%), or a Fraction (3/4).

Probability is determined by the following Equation:

PROBABILITY = Number of times an event is expected to happen

Probability Examples

With each coin flip there is a 50% chance of heads and 50% chance of tails.

Chance of inheriting one of two alleles from a parent is also 50%.

Each coin toss is an independent event. Therefore, the probability of flipping three heads in a row is: ½ X ½ X ½ = 1/8

Rules of Probability

Offspring resemble their parentsWhy ?

• What is transmitted from parent to offspring ?

• Offspring are not all identical

• Why ?

• Inheritance is often discrete, not blending

• Why ?

• Acquired characters are not inherited

• Why ?

Mendel said that Blending Inheritance does not occur

• Blending implies that offspring have a simple mixture of the parent’s characteristics

• Mendel showed that characteristics from each parent are separate and that offspring inherit the characteristics of one or other parent depending on certain rules of inheritance

Mendel’s experimental design

1) Produced true-bred pea plants by self-fertilization for several generations. Thus, he assured that traits were constant, and transmitted unchanged

from generation to generation. 2) He then performed crosses between varieties

exhibiting alternative forms of traits. e.g. crossed white flower on a plant with a plant that produced purple flowers.

3) He permitted the hybrid offspring to self-fertilize for several generations. Thus, he allowed the alternative forms of a character to segregate among the offspring (progeny).

What Mendel found

The F1 generation

True-breeding parents with contrasting forms of a trait

• When Mendel crossed 2 contrasting varieties of peas, the hybrid offspring did not have flowers of intermediate color as the theory of blending inheritance would predict.

• Instead, the flower color always resembled one of the parents.

F1 or first filial generation

• It is customary to refer to these offspring as the F1 or first filial (filius is Latin for son) generation.

• Mendel referred to the trait expressed in the F1 plants as dominant and the alternative form as recessive.

• We usually indicate the dominant allele with an upper case character (e.g. A) and the recessive with a lower case character (e.g. a)

The F2 generation

• He allowed individual F1 plants to self-fertilize, he found that 705 F2 plants (75.9% had purple flowers and 224 (24.1%) had white flowers.

• Approximately, 1/4 exhibited the recessive form.

• In other words, the dominant: recessive ratio was 3 purple:1 white.

Punnett Squares

• ,

R.C. Punnett developed a simple diagram (now called a Punnett Square) for visualizing the possible genotypic combinations of F2 individuals

F2 generation

• The heterozygous Aa plants look just like the homozygous AA plants (i.e. have the same phenotype)

• Thus, the 3:1 phenotype ratio is a disguised 1AA:2Aa:1aa = 1:2:1 genotype ratio

Mendel’s results• No blending• 3:1 phenotypic ratio• 1:2:1 genotypic ratio• Punnett square shows that each sperm has

equal probability of fertilizing an egg.• Therefore, each plant has a 3/4 (75%)

chance of inheriting one dominant allele

Monohybrid Cross

Mendel’s First Law of Heredity: Segregation

• Each organism contains two factors (alleles) for each trait. These factors segregate, or separate, in the formation of gametes (eggs and sperm).

• This segregational behavior has a simple physical basis: the alignment of chromosomes are random on the metaphase plate during meiosis

Mendel’s Second Law of Heredity: Independent Assortment

• Mendel then set out to test whether different genes segregate independently.

• He tested a Dihybrid case (RrYy X RrYy):• In a cross involving different seed shape alleles

(round R and wrinkled r) and different seed color alleles (yellow Y and green y), all the F1 individuals were identical, each one heterozygous for both seed shape (Rr) and seed color (Yy).

Dihybrid Cross

What did Mendel actually observe ?

• From a total of 556 seeds from dihybrid plants that he allowed to self-fertilize he observed:

• 315 round yellow (R_Y_), 108 round green (R_yy), 101 wrinkled yellow (rrY_) and 32 wrinkled green (rryy).

• These results are very close to a 9:3:3:1 ratio (would be 313:104:104:35).

• Consequently, the 2 genes appeared to assort independently of each other.

Mendel’s Second Law of Heredity:

Independent Assortment

• A modern re-statement of this 2nd law is:

Genes that are located on different chromosomes assort independently during meiosis.

The mechanism of Independent Assortment

Because…….

It is a fundamental law of probability that the probability of

two independent events occurring together is the product

of their individual probabilities

This is the Law of Multiplicative Probabilities

Multihybrid = n loci• For example, cross 5 independent loci:

• Aa bb Cc Dd Ee X Aa Bb Cc dd Ee

• What is the proportion of homozygous recessives (aa bb cc dd ee) ?

• Aa X Aa: ½ a X ½ a = ¼ aa• bb X Bb: 1/1 b X ½ b = ½ bb• Cc X Cc: ½ c X ½ c = ¼ cc• Dd X dd: ½ d X 1/1 d = ½ dd• Ee X Ee: ½ e X ½ e = ¼ ee

• Multiply probabilities: ¼ X ½ X ¼ X ½ X ¼ = 1/256 (=0.39%)

Gene Recombination• Now we know that a gene codes for a protein (enzyme)• Genetic Recombination – when there is a new

combination of genes produced by crossing over.– Linked genes usually travel together during gamete formation.

This is an exception to Mendel’s law of segregation.– Crossing over is more frequent between genes that are far

apart than close together. This information can be used to develop a chromosome map which maps the sequence of genes on a chromosome

• Polyploidy – one or more extra set of chromosomes (not diploid)– Example: Triploid(3N)- found in goldfish and earthworms;

lethal in humans

Exceptions1. incomplete dominance – occurs when Two or More Alleles Influence the

Phenotype, resulting in a Phenotype Intermediate between the Dominant Trait and Recessive Example red X white 4 o’clock flowers = pink flowers

2. codominance – occurs when Both Alleles for a gene are Expressed in a Heterozygous offspring

Examples: certain varieties of chicken where black X white = speckled chicken Sickle Cell Anemia

3. multiple alleles – more than 2 possible alleles for a gene. Examples: blood type in humans, rabbit coat color 4. polygenic traits – traits controlled by 2 or more genes. Examples: eye color of fruit flies, human skin color5. Sex Determination – sex chromosomes (X and Y) vs autosomes

Sex-Linked Traits – recessive X linked trait Examples: color blindness, hemophilia

X-Linked Color Blindness

Polygenic Inheritance