Introduction to Mendelian GeneticsMonohybrid Crosses

Lesson 4.

Learning goals

• Understand the basic terminology of mendelian genetics, not limited to (phenotype, genotype, homozygous, heterozygous, P,F1,F2 generation(s), dominant, recessive, alleles, gene

• Solve monohybrid problems using Punnett square method

Gregor Mendel (1822-1884)• Used pea plants to study

genetics, heredity and variation.

• Looked at 7 different traits that only had two variations.

• From his meticulous work he came up with many “key terms” and, more importantly, two generalizations that later became known as Mendel’s Laws.

1677 Anton van Leewenhoek

• First discovered living sperm (also designed first microscope)

• Believed he saw a complete miniature person in the head of a sperm; homunculi

1919thth Century British Century British

• From breeding ornamental plants, scientists believed in a blending theory of inheritance.

• Charles Darwin – theorized that offspring have variations of their parents' characteristics, but he was unable to explain why (1870s).

Gregor Mendel (1822-1884)Gregor Mendel (1822-1884)• Austrian monk

• Considered the father of genetics (now aka Mendelian Genetics)

• Why the pea plant?– grows easily and quickly – has seven traits that are easily

identified – it is easy to manipulate

7 traits Mendel looked at7 traits Mendel looked at

Mendel’s true breeding experiments

• Pea plants reproduce sexually, but usually self fertilize

• True breeding: organisms that exhibit the same traits generation after generation

Mendel's Pea Plant ExperimentMendel's Pea Plant Experiment• Mendel selected true bred plants for different traits and

cross-fertilized them to see what would happen.

• True breeding: Individuals that only contain one variation of a trait and therefore can only pass this one variation on to future generations. We now call these individuals homozygous, or having two alleles that are the same.

• For example:– Tall x dwarf

• What do you think the offspring looked like?X

• 100% of the offspring plants were tall! (Instead of a blending – where all plants would be medium sized...)

X

x

• Mendel then let the F1 generation of offspring self-pollinate...

• F1 generation – (filial (son or daughter) generation) offspring from the cross (reproduction) of the Parent or P generation.

• this time most were tall, but some were short and ALWAYS 3:1 ratio

Why?Why?• Each plant in the F1 generation carried an allele from the P

generation; a tall allele and a dwarf allele. They are what are called hybrids.

• Hybrids: Individuals that contain more than one variation of a trait and therefore can pass on more than one variation to future generations. We now call these individuals heterozygous, or having two alleles that are different from each other.

• Traits are dominant or recessive. If a dominant trait is inherited, it will be expressed.

• What is the dominant trait for height in pea plants?

Mendel’s “Key Terms”• Phenotype: the appearance of traits in an organism (eg.

short or tall)

• Genotype: the specific genes that an organism has (which alleles, dominant or recessive)

• Every individual has a phenotype and genotype for every trait.

• The phenotype is easy to figure out, but occasionally the genotype is hard to determine, especially for dominant traits.

Alleles: Alternate forms of a gene

Genotype vs. Phenotype

Which Allele is expressed?

Can you tell the genotype of an individual by their phenotype?

Mendel's Laws and PrinciplesMendel's Laws and Principles

• Law of Segregation: when any individual produces gametes, the copies of a gene separate so that each gamete receives only one copy of a gene and therefore only one allele for all possible traits.

Principle of Segregation Demystified

The principle of segregation is explained by the behavior of homologous chromosomes at meiosis.

Segregation

Law of Segregation

• Law of Independent Assortment: alleles of different genes assort independently of one another during gamete formation, so different traits are inherited independently of each other

• In plain English, this means that if you are able to be dominant for one trait, it will have no influence on whether you will be dominant for another trait.

http://www.youtube.com/watch?v=mx6maAOOfxQ

Punnett Square

Punnett Square

Dominant traits

This phenomenon can be explained by thefact that some ________ are dominant. Adominant allele is an allele that __________its phenotypic effect when present in theindividual.

Recessive traits

9. A recessive allele is expressed only when _____ alleles are of the recessive form. The genotype of an individual showing a recessive phenotype must be ____________ for the recessive allele.

Punnett Square

Punnett Square

Genotypic proportions

11. The genotypic proportions of the offspring from the cross of two heterozygous parents (Bb) are:

• BB __/4• Bb __/4• bb __/4

Phenotypic proportions

12. The phenotypic proportions of the offspring are:

• Purple __/4 or __%• White __/4 or __%

Monohybrid cross problem

Curly hair is dominant over straight hair. If amother is homozygous dominant for curlyhair and the father has straight hair, whattype of hair will the children have?

Test Crosses

• When geneticists want to know if an individual is heterozygous or homozygous– Why? For breeding purposes

• Cross between unknown phenotype and homozygous recessive individual– (Pp or PP) x pp

• Pp x pp = 50% purple, 50% white• PP x pp = 100% purple