Introduction to Genetics

How are traits passed from parent to offspring?

• Traits– Traits are physical or physiological characteristics of an organism

– Ex. Height or blood type

– http://www.pbs.org/wgbh/nova/body/science-picky-eaters.html

How are traits passed from parent to offspring?

• Traits– The term phenotype is used to describe the physical expression of the trait

– Ex. Short/tall or Type A/Type B

Chemical basis of traits

• DNA is the molecule that contains the information to make proteins, which control our traits

Chemical basis of traits

• A section of DNA that is used to make a protein is called a gene.

• There are many genes (hundreds) on a single chromosome.

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How are the following terms related?

gene, DNA, chromosome

Chemical basis of traits

• Eukaryotic organism’s chromosomes exist in pairs.

• One is inherited from the sperm and one is inherited from the egg.

• Each chromosome, in a pair, contains genes for the same traits.

• This is why we call them homologous pairs.

Chemical basis of traits

• Although the genes on homologous chromosomes may code for the same trait, slight differences in the DNA sequences may lead to different forms of the protein.

Chemical basis of traits

• This creates slightly different versions of the same trait.

• Each version is called an allele.– Ex. Blue and brown

are two alleles of the eye color trait.

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• Draw a homologous pair of chromosomes showing a different allele for eye color on each:

Chemical basis of traits

• Genotype is the term used to describe the combination of alleles present in an organism’s chromosomes.

• An allele is usually represented by a single letter.

• Thus a genotype is usually represented by two letters.

Chemical basis of traits• If an individual

inherits identical copies from each parent the individual is considered homozygous (pure breeding)– Ex. AA or aa

• If an individual inherits a different copy from each parent the individual is considered heterozygous (hybrid)– Ex. Aa

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WORD HELP:Pheno - ________________

Geno - _________________

Homo - _________________

Hetero - _________________

Check Yourself!1. Give an example (not in the notes) of a

human trait.

2. What is a phenotype?

3. From where did you get each of the chromosomes in the homologous pair?

4. What is an allele?

5. What is a genotype?

Check Yourself!1. Give an example (not in the notes) of a

human trait. ANSWERS WILL VARY

2. What is a phenotype?

3. From where did you get each of the chromosomes in the homologous pair?

4. What is an allele?

5. What is a genotype?

Check Yourself!1. Give an example (not in the notes) of a

human trait. ANSWERS WILL VARY

2. What is a phenotype? PHYSICAL EXPRESSION OF A TRAIT

3. From where did you get each of the chromosomes in the homologous pair?

4. What is an allele?

5. What is a genotype?

Check Yourself!1. Give an example (not in the notes) of a

human trait. ANSWERS WILL VARY

2. What is a phenotype? PHYSICAL EXPRESSION OF A TRAIT

3. From where did you get each of the chromosomes in the homologous pair? ONE FROM MOM AND ONE FROM DAD

4. What is an allele?

5. What is a genotype?

Check Yourself!1. Give an example (not in the notes) of a

human trait. ANSWERS WILL VARY

2. What is a phenotype? PHYSICAL EXPRESSION OF A TRAIT

3. From where did you get each of the chromosomes in the homologous pair? ONE FROM MOM AND ONE FROM DAD

4. What is an allele? DIFFERENT VERSIONS OF A PARTICULAR TRAIT

5. What is a genotype?

Check Yourself!1. Give an example (not in the notes) of a

human trait. ANSWERS WILL VARY

2. What is a phenotype? PHYSICAL EXPRESSION OF A TRAIT

3. From where did you get each of the chromosomes in the homologous pair? ONE FROM MOM AND ONE FROM DAD

4. What is an allele? DIFFERENT VERSIONS OF A PARTICULAR TRAIT

5. What is a genotype? COMBINATION OF ALLELES FOR AN ORGANISM

How was the path of inheritance discovered

…• in a garden in

Austria?• Gregor Mendel

– The father of genetics

• Using pea plants, Mendel proved experimentally the link between meiosis, genes, and inheritance (long before we knew about DNA!)

• He developed 3 basic conclusions:

The Principle of Dominance

• Certain alleles (forms of a trait) can hide/mask other alleles.

• These alleles are called dominant alleles and are represented by a capital letter (A).

• The alleles that may be hidden are called recessive alleles and are represented by the lower case of the same letter (a)

The Principle of Dominance

• Thus, homozygous dominant is AA and will express the dominant phenotype.

• Heterozygous individuals are Aa and will also express the dominant phenotype.

• Only homozygous recessive (aa). individuals will express the recessive phenotype

Fill It In …

Draw a picture of a dominant and recessive trait. Write the possible genotypes under each:

The Principle of Segregation

• Alleles are not passed in pairs from one parent to an offspring.

• Each parent only donates half of each offspring’s genotype (typically one allele per trait).

The Principle of Segregation

• This is because during meiosis only one of each homologous pair of chromosomes is passed to the gamete (sperm or egg)

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Based on the parent cell:

What are the possible gametes (after segregation in meiosis)?

A a

The Principle of Independent Assortment

• The way one pair of chromosomes is separated during meiosis does not affect the way the next pair separates.

The Principle of Independent Assortment

• A gene for one trait is only passed in connection with a gene for a different trait if the two genes are on the same chromosome.

• Genes on separate chromosomes are passed independently of each other.

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Mendel’s Three Laws:

1. D_______________

2. S_______________

3. I_______________ A_______________

Check Yourself!1. Name Mendel’s three principles of

heredity:

2. What cell process allows the principle of segregation to take place?

3. When would a gene for one trait be passed with a gene for a different trait?

Check Yourself!1. Name Mendel’s three principles of

heredity: PRINCIPLE OF DOMINANCE PRINCIPLE OF SEGREGATION PRINCIPLE OF INDEPENDENT ASSORTMENT

2. What cell process allows the principle of segregation to take place?

3. When would a gene for one trait be passed with a gene for a different trait?

Check Yourself!1. Name Mendel’s three principles of

heredity: PRINCIPLE OF DOMINANCE PRINCIPLE OF SEGREGATION PRINCIPLE OF INDEPENDENT ASSORTMENT

2. What cell process allows the principle of segregation to take place? MEIOSIS

3. When would a gene for one trait be passed with a gene for a different trait?

Check Yourself!1. Name Mendel’s three principles of

heredity: PRINCIPLE OF DOMINANCE PRINCIPLE OF SEGREGATION PRINCIPLE OF INDEPENDENT ASSORTMENT

2. What cell process allows the principle of segregation to take place? MEIOSIS

3. When would a gene for one trait be passed with a gene for a different trait?

INDEPENDENT ASSORTMENT