Why are you unique? 

• Why do you resemble your parents, but not exactly?

• How does the DNA from your parents control your traits?

Gregor Mendel

Background Information

• Mendel lived in Austria in the 1800s and studied math, science, and religion before becoming a monk and school teacher.

• He grew a garden in the monastery and used the pea plants in a famous experiment.

• He controlled the reproduction of the pea plants and observed the inheritance patterns of certain traits. 

• From his experiments he made two famous conclusions which we now call Mendel's Laws of Inheritance.

Vocabulary:

• cross: sexual reproduction between two organisms
• example: "Mendel crossed a purple flowered pea plant with a white flowered pea plant."

• inheritance (noun) / inherit (verb): passing traits from parents to offspring.
• example: "Mendel studied the inheritance of flower color traits in pea plants."

• trait: a characteristic of an organism that is passed on (inherited) from parents to offspring. 
• example: "Traits are controlled by an organism's genes."

• allele: a specific version of a gene.
• example: "Mendel's pea plants have genes for flower color that can be either the purple allele or the white allele."

• genotype: the two alleles for a gene on homologous chromosomes.
• example: "A diploid cell has two alleles fro a trait."

• phenotype: the trait expressed by the genotype.
• example: "The pea plant genotype is two white alleles for flower color, so it has the phenotype of white flowers."

http://stu.westga.edu/~kkilken1/curriculum_web/mendel.html

https://hopes.stanford.edu/sites/hopes/files/f_b11homolgs.jpg

Mendel's Experiment

• Mendel crossed a purple flowered pea plant with a white flowered pea plant.

• P generation: Purple and White

• First cross: Purple X White 

• All the offspring had purple flowers

• First generation (F₁): 100% Purple

• Mendel crossed two purple flowered plants from the F₁ generation.

• Second cross: Purple X Purple

• Second generation (F₂): 75% Purple and 25% White

• The offspring were 75% purple flowered plants and 25% white flowered plants (ratio 3:1).

http://www.bioinformatics.nl/webportal/background/mendelinfo.html 

Traits Studied by Mendel

Mendel repeated the experiment many times and also observed other traits.

• seeds: smooth or wrinkled

• seeds: yellow or green

• flowers: axial or terminal 

• flowers: purple or white 

• pea pod: inflated or constricted

• pea pods: green or yellow

• plant height: tall or short 

All the traits showed the same pattern of inheritance:

• P = two different traits

• F₁ = only one trait

• F₂ = two traits in a ratio of 3:1

Why?

Results of Mendel's Experiments

http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20101/bio%20101%20lectures/genetics-%20genes/mendelia.htm

http://activity.ntsec.gov.tw/lifeworld/english/content/gene_cc4.html

Mendel's Conclusions

Mendel saw the same pattern of inheritance for many traits in the pea plants.

• Only one parental trait was seen in the F₁ generation, then both traits appeared in the F₂ generation in a ratio of 3 to 1.

What caused this?

Mendel concluded that each plant had two "factors" that controlled the trait.

Mendel's factors are alleles.

Each trait is controlled by two alleles on homologous chromosomes.

An allele could be dominant or recessive:

• dominant: the trait is always expressed.
• recessive: the trait is only expressed when both alleles are recessive.

A plant could have two dominant alleles, two recessive alleles, or one dominant and one recessive allele. 

The genotype is the two alleles for a gene/trait.

• homozygous: two of the same allele for a genotype.
• heterozygous: two different alleles for a genotype.

Letters are used to represent the alleles of a gene.

• Big letters fro dominant alleles.
• Small letters for recessive alleles.

For example "P" and "p" can represent puprle and white flower color.

Genotypes:

• PP = homozygous dominant
• Pp = heterozygous
• pp = homozygous recessive

The phenotype is the appearance of the trait in the organism.

Genotypes and Phenotypes:

• PP = purple flowers
• Pp = purple flowers
• pp = white flowers

Mendel's Law of Segregation

For each trait, an organism has two alleles. One allele came from the mother; one allele came from the father. An organism passes on only one allele to an offspring, 

Flower Color Experiment

http://www.anselm.edu/homepage/jpitocch/genbio/geneticsnot.html

Plant Height Experiment 

http://activity.ntsec.gov.tw/lifeworld/english/content/gene_cc4.html

Homework

1. Use the term cross in a sentence.

2. Define inheritance in your own words.

3. Use the term inherit in a sentence.

4. List two examples of human traits.

5. Use the terms gene and allele in one sentence that explains their relationship.

6. Use the terms genotype and phenotype in one sentence that explains their relationship.

7. Compare the terms dominant and recessive.

8. Compare the terms heterozygous and homozygous. 

9. Compare the terms homozygous dominant and homozygous recessive.

Part 2: Mendel's Experiment

Draw a diagram of Mendel's experiment.

• Choose traits for the parents. Don't use flower color.
• Draw a picture of the traits of the parents.
• Label the phenotypes and genotypes.
• Make a Punnett Square for the parental cross.
• Draw a picture of the offspring's traits.
• Label the phenotypes and genotypes.
• Write ratios or percentages of the phenotypes and genotypes. 
•  Draw a picture of the traits of the F₁ cross. 
• Make a Punnett Square for the F₁ cross.
• Draw a picture of the offspring's traits.
• Label the phenotypes and genotypes.
• Write ratios or percentages of the phenotypes and genotypes.

Part 3: Punnett Square Worksheet 

http://cccmkc.edu.hk/~kei-kph/Mendelian%20genetics/Dihybrid%20inheritance_TtPp%20punnett%20square_chalkboard.htm

Mendel's Laws

The Law of Segregation

• two alleles control a trait
• only one of the alleles is passed on to offspring
• the offspring receives one allele from each parent for a total of two alleles

The Law of Independent Assortment

• Different traits are inherited independently.
• One trait does not affect another trait.

For example, pod color and pod shape are inherited independently.

• Pod color can be green or yellow
• Pod shape can be wrinkled or round.
• If two plants with green and wrinkled pods are crossed....
• parents genotypes = RrYy
• Cross: RrYy x RrYy 
• Offspring genotypes =
• green and wrinkled
• green and round
• yellow and wrinkled
• yellow and round 

Mendel studied the inheritance of two traits, and he saw that they do not affect each other. 

Look at the diagram below.

• It shows two cells from the same parent going through meiosis.
• Segregation happens when the chromosomes are separated into haploid cells.
• Independent Assortment happens becasue the chromosome pairs line up randomly in metaphase I.
• As the two cells go through meisosis, the chromosomes line up differently and segregate into gametes.
• There are four different gametes produced.
• Original cell = RrYy
• Gametes = RY   Ry   rY   ry

Sex-Linked Traits

Some traits are not inherited like the traits of Mendel's peas.

Sex-linked traits are an example of non-Mendelian inheritance.

Sex-linked traits show different inheritance patterns in males and females. 

Sex-linked traits are controled by alleles on the sex chromosomes.

Humans have 23 pairs of chromosomes.

The 1st through 22nd parir are called autosomes.

The 23rd pair are called sex chromosomes.

• One is called "X"
• The other is called "Y"

A human female has two X chromosomes.

A human male has one X and one Y chromosome.

• Female = XX
• Male = XY

Besides determining male and female chracteristics, other traits are controlled by genes onthe sex chromosomes.

For example, a gene that controls vision is on the X chromosome.

The dominant allele makes normal vision.

The recessive allele makes colorblind vision.

Colorblindness is the inability to distinguish some colors, like red and green look the same.

C = normal vision

c = colorblind vision

For sex-linked traits, write the chromosomes and the genotypes as superscripts:

X^CX^C = normal

X^CX^c = normal; "carrier"

X^cX^c = colorblind

X^CY =  normal

X^cY = colorblind

Notice that males only have one X chromosome, so their phenotype is controlled by only one allele.

Males have a higher chance if having colorblindness because they only have one X chromosome.

The Y chromosome does not have the smae genes as the X chromosome.

The Y and X Chromosomes 

        XX x XY

50% male offspring, 50% female offspring

Crosses with Sex-Linked Traits

Pedigrees

pedigree: a diagram that shows the inheritance pattern of a trait over generations.

Symbols: 

Pedigree Activity - click to go to an interactive pedigree activity

• drag-and-drop pedigree only works with Internet Explorer
• alternatively, you can draw the pedigrees on paper

Read the instructions in the Gameplay Tips on the Overview page.

Draw pedigrees for theTongue Rolling and Eye Color traits.

Example: Pedigree of a sex-linked trait

http://www.macalester.edu/academics/psychology/whathap/ubnrp/visionwebsite04/twotypes.html

Pedigree Quiz (retreived from http://www.felloweducators.com/brick/gen/gena_pedigree_quiz_ib.htm)

Use the pedigrees below to answer these questions. The shaded symbols represent individuals with a trait.

Pedigree 1 - polydactyl trait (6 fingers)

1. How many generations are shown ? _______
2. How many male children did couple A-B have ? ______
3. List the aunts that child J has : _______________
4. What is the relationship between I and K ? __________
5. What is the relationship between A-B and I, J and K ? __________
6. Can an affected individual have a parent who does not have the trait ?
7. Let us assign the letter 'P' for the dominant allele and 'p' for the recessive allele. How many 'P' alleles does an individual need to express the trait ?
8. Is this a dominant or recessive trait ?
9. What must be the genotype for B ?
10. If A's genotype is 'Pp' and B's is 'pp', use a Punnett square to show the genotypes of their offspring.
11. What is the percentage that any of A-B's children would be polydactyl ?
12. Use a Punnett square to show that A's genotype must be 'Pp', not 'PP'.

Pedigree 2 - attached earlobes trait

1. How many generations are shown ? _______
2. How many male children did couple A-B have ? ______
3. List the aunts that child J has : _______________
4. What is the relationship between I and C ? __________
5. What is the relationship between A-B and J, G and H ? __________
6. Can an affected individual have a parent who does not have the trait ?
7. Let us assign the letter 'F' for the dominant allele and 'f' for the recessive allele. How many 'f' alleles does an individual need to express the trait ?
8. Is this a dominant or recessive trait ?
9. What must be the genotype for A ?
10. If A's genotype is 'ff' and B's is 'FF', use a Punnett square to show the genotypes of their offspring.
11. What is the percentage that any of A-B's children would express attached earlobes ?
12. Use a Punnett square to show that B's genotype must be 'FF', not 'Ff'.