Mendelian Genetics

An Overview

Vocabulary

• Genetics: The scientific study of heredity

• Character: heritable feature

• Trait: each variant for a character

• True-breeding: plants that self-pollinate all offspring are the same variety

• Allele: alternate version of a gene

• Dominate allele: An allele which is expressed (masks the other) in the heterozygote & homozygote

• Recessive allele: An allele which is present but remains unexpressed (masked) in the heterozygote

Vocabulary (continued)• Homozygote – pair of identical alleles for a character

– Homozygous dominant- BB

– Homozygous recessive - bb

• Heterozygote – two different alleles for a character (Bb)

• Genotype – genetic makeup; combination of alleles an organism has

• Phenotype – appearance of an organism; the characteristics determined by the genotype and environmental influences

Vocabulary

• Monohybrid cross – a cross that tracks the inheritance of a single character

• P generation – (parental) true-breeding

• F1- (first filial) offspring of P generation

• F2 – (second filial) offspring from F1 cross

HistoryHistory

• Principles of genetics were developed in the mid 19th century by Gregor Mendel an Austrian Monk

• Developed these principles without ANY scientific equipment - only his mind.

• Experimented with pea plants, by crossing various strains and observing the characteristics of their offspring.

• Studied the following characteristics:– Pea color (Green, yellow)– Pea shape (round, wrinkled)– Flower color (purple, white)– Plant height (tall, short)

• Made the following observations (example given is pea shape)

• When he crossed a round pea and wrinkled pea, the offspring (F1 gen.) always had round peas.

• When he crossed these F1 plants, however, he would get offspring which produced round and wrinkled peas in a 3:1 ratio.

• Pea plants have several advantages for genetics.– Pea plants are available in many varieties with distinct

heritable features (characters) with different variants (traits).

– Another advantage of peas is that Mendel had strict control over which plants mated with which.

– Each pea plant has male (stamens) and female (carpal) sexual organs.

– In nature, pea plants typically self-fertilize, fertilizing ova with their own sperm.

– However, Mendel could also move pollen from one plant to another to cross-pollinate plants.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 14.1

• In a typical breeding experiment, Mendel would cross-pollinate (hybridize) two contrasting, true-breeding pea varieties.– The true-breeding parents are the P generation and

their hybrid offspring are the F1 generation.

• Mendel would then allow the F1 hybrids to self-pollinate to produce an F2 generation.

• It was mainly Mendel’s quantitative analysis of F2 plants that revealed the two fundamental principles of heredity: the law of segregation and the law of independent assortment.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Laws of Inheretance

• Law of Segregation: When gametes (sperm, egg, etc…) are formed each gamete will receive one allele or the other.

• Law of Independent Assortment: Two or more alleles will separate independently of each other when gametes are formed

• If the blending model were correct, the F1 hybrids from a cross between purple-flowered and white-flowered pea plants would have pale purple flowers.

• Instead, the F1 hybrids all have purple flowers, just a purple as the purple-flowered parents.

By the Law of Segregation, the two alleles for a characters are packaged

into separate gametes

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 14.2

Punnett Squares• Genetic problems can be easily solved using a

tool called a Punnett square.– Tool for calculating genetic probabilities

A Punnett square

Monohybrid cross (cross with only 1 trait)

• Problem:

• Using this is a several step process, look at the following example– Tallness (T) is dominant over shortness (t) in pea plants.

A Homozygous tall plant (TT) is crossed with a short plant (tt). What is the genotypic makeup of the offspring? The phenotypic makeup ?

Punnet process

1. Determine alleles of each parent, these are given as TT, and tt respectively.

2. Take each possible allele of each parent, separate them, and place each allele either along the top, or along the side of the punnett square.

Punnett process continued

• Lastly, write the letter for each allele across each column or down each row.

• The resultant mix is the genotype for the offspring.

• In this case, each offspring has a Tt (heterozygous tall) genotype, and simply a "Tall" phenotype.

Punnett process continued• Lets take this a step further

and cross these F1 offspring (Tt) to see what genotypes and phenotypes we get.

• Since each parent can contribute a T and a t to the offspring, the punnett square should look like this…

Punnett process continued

• Here we have some more interesting results: First we now have 3 genotypes (TT, Tt, & tt) in a 1:2:1 genotypic ratio. We now have 2 different phenotypes (Tall & short) in a 3:1 Phenotypic ratio. This is the common outcome from such crosses.

• When Mendel allowed the F1 plants to self-fertilize, the F2 generation included both purple-flowered and white-flowered plants.– The white trait, absent in the F1, reappeared in the

F2.

• Based on a large sample size, Mendel recorded 705 purple-flowered F2 plants and 224 white-flowered F2 plants from the original cross.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 14.2

Law of Segregation - the two alleles for each character segregate during gamete production

Law of Independent Assortment – Each set of alleles segregates independently

Test cross – designed to reveal the genotype of an organism

Mendelian Inheritance and Rules of Probability

• Rule of Multiplication – the probability that two events will occur simultaneously is the product of their individual probabilities

• Probability that an egg from the F1 (Pp) will receive p = ½

• Probability that an sperm from the F1 (Pp) will receive p = ½

• Probability that a of offspring receiving two recessive alleles during fertilization½ x ½ = ¼

Rule Applies to dihybrid Crosses

• For a dihybrid cross, YyRr x YyRr, what is the probability of an F2 having the genotype YYRR?

• Go page 267 and work #9 and #10

Dihybrid Crosses

• Dihybrid crosses are made when phenotypes and genotypes composed of 2 independent alleles are analyzed.

• Process is very similar to monohybrid crosses.• Example:

– 2 traits are being analyzed

– Plant height (Tt) with tall being dominant to short,

– Flower color (Ww) with Purple flowers being dominant to white.

Dihybrid Cross Example• The cross with a pure-breeding (homozygous)

Tall,Purple plant with a pure-breeding Short, white plant should look like this.

F1 generation

Dihybrid Cross Example continued

• Take the offspring and cross them since they are donating alleles for 2 traits, each parent in the f1 generation can give 4 possible combination of alleles. TW, Tw, tW, or tw. The cross should look like this. (The mathematical “foil” method can often be used here)

F2 Generation

Dihybrid Cross Example continued

• Note that there is a 9:3:3:1 phenotypic ratio. 9/16 showing both dominant traits, 3/16 & 3/16 showing one of the recessive traits, and 1/16 showing both recessive traits.

• Also note that this also indicates that these alleles are separating independently of each other. This is evidence of Mendel's Law of independent assortment

Other Factors: Incomplete Dominance

• Some alleles for a gene are not completely dominant over the others. This results in partially masked phenotypes which are intermediate to the two extremes.

Incomplete Dominance

Codominance

• Two alleles affect the phenotype in separate and distinguishable ways.

• Neither allele can mask the other and both are expressed in the offspring and not in an “intermediate” form.

• Example: red flowers that are crossed with white flowers that yield red and white flowers.

• 1) In cattle, roan coat color (mixed red and white hairs) occurs in the heterozygous (Rr) offspring of red (RR) and white (rr) homozygotes. When two roan cattle are crossed, the phenotypes of the progeny are found to be in the ratio of 1 red:2 roan:1 white. Which of the following crosses could produce the highest percentage of roan cattle?

• A) roan x roan • B) red x white • C) white x roan • D) red x roan • E) All of the above crosses would give the same

percentage of roan.

Multiple Alleles

Page 267 and work #6

Pleiotropy

• Most genes have multiple phenotypic effects. The ability of a gene to affect an organism in many ways is called pleiotropy.

Epistasis

• Epistasis occurs when a gene at one locus alters or influences the expression of a gene at a second loci. In this example, C is for color and the dominate allele must be present for pigment (color) to be expressed.

Polygenetic Inheritance• Qualitative variation

usually indicates polygenic inheritance. This occurs when there is an additive effect from two or more genes. Pigmentation in humans is controlled by at least three (3) separately inherited genes.

Other Factors: Continuous Variation

• Many traits may have a wide range of continuous values. Eg. Human height can vary considerably. There are not just "tall" or "short" humans

Chromosomes and Classical Genetics

• Walter Sutton in 1902 proposed that chromosomes were the physical carriers of Mendel's alleles

• Problems arose however regarding the following question:

• Why are the number of alleles which undergo independent assortment greater than the number of chromosomes of an organism?

• This was explained understanding of 2 additional factors; Sex Linkage and crossing over

Sex Linkage

• All chromosomes are homologous except on sex chromosomes.

• Sex chromosomes are either X or Y.• If an organism is XX, it is a female, if XY it is

male.• If a recessive allele exists on the X chromosome.

It will not have a corresponding allele on the Y chromosome, and will therefore always be expressed

Sex Linkage Example• Recessive gene for white eye

color located on the Xw chromosome of Drosophila.

• All Males which receive this gene during fertilization (50%) will express this.

• If a female receives the Xw chromosome. It will usually not be expressed since she carries an X chromosome with the normal gene

Human Sex Linkage

• Hemophilia:– Disorder of the blood where

clotting does not occur properly due to a faulty protein.

– Occurs on the X chromosome, and is recessive.

• Thus a vast majority of those affected are males.

– First known person known to carry the disorder was Queen Victoria of England. Thus all those affected are related to European royalty.

Hemophilia and Royalty

Other Factors: Multiple Alleles• Phenotypes are controlled by more than 1 allele. Eg. Blood types are

regulated by 3 separate genes.

• ABO Blood typing– Humans have multiple types of surface antigens on RBC's – The nature of these surface proteins determines a person's Blood Type.– There are 3 alleles which determine blood type IA, IB, or IO. This is

referred to as having multiple alleles– Human blood types are designated as A, B or O.

• Type A denotes having the A surface antigen, and is denoted by IA

• Type B denotes having the B surface antigen, and is denoted by IB

• Type O denotes having neither A or B surface antigen, and is denoted by IO

– There are several blood type combinations possible• A• B• AB (Universal recipient)• O (Universal donor)

Blood & Immunity• A person can receive blood only when the donor's blood type does not

contain any surface antigen the recipient does not. This is because the recipient has antibodies which will attack any foreign surface protein.

• Thus, Type AB can accept any blood types because it will not attack A or B surface antigens. However, a type AB person could only donate blood to another AB person. They are known as Universal Recipients.

• Also, Type O persons are Universal donors because they have NO surface antigens that recipients' immune systems can attack. Type O persons can ONLY receive blood from other type O persons.

• There is another blood type factor known as Rh.• People are either Rh+ or Rh- based on a basic dominant/recessive

mechanism.• Not usually a problem except with pregnancy.• It is possible that an Rh- mother can carry an Rh+ fetus and develop

antibodies which will attack & destroy the fetal blood• This usually occurs with 2nd or 3rd pregnancies, and is detectable and

treatable.

Other Factors

• Gene interaction: – Many biological pathways are governed by multiple

enzymes, involving multiple steps. If any one of these steps are altered. The end product of the pathway may be disrupted.

• Environmental effects:– Sometimes genes will not be fully expressed owing to

external factors. Example: Human height may not be fully expressed if individuals experience poor nutrition.

Environmental Impact on Phenotype

pH of the soil will change the color of hydrangea flowers from blue to pink

The The Average Average

American American PhenotypePhenotype

Technology

And

Genetic testing

Carrier Recognition

Newborn screening

Fetal testing

1. Amniocentesis

2. Chorionic villus sampling (CVS)

3. Ultrasound

4. Fetoscopy

Mendelian Genetics

End