Genetics of Reproduction Notes

The Genetics of Reproduction• Organisms can either reproduce

asexually or sexually– Asexual reproduction = 1 parent

• Since there is only 1 parent, the offspring is genetically identical to the parent (no new DNA combinations)

– Sexual reproduction = 2 parents• “Males” produce sex cells called sperm

– Half of the “father’s” DNA is in this sperm• “Females” produce sex cells called eggs

– Half of the “mother’s” DNA is in this egg• Eggs and sperm combine in an event

called fertilization which creates the new offspring organism

– The offspring is gets half its DNA from its “mother” and half from its “father”

– This results in offspring that are genetically similar to the parents, but not identical (new DNA combination)

DNA in a normal cell• In a normal cell, humans will have 46 chromosomes

– Chromosomes are long, continuous strands of DNA that contain both genes and introns

– This number is different for organisms that belong to different species

• 23 of those chromosomes came from the mother’s egg– 1 of each numbered chromosome and 1 sex chromosome

• 23 of those chromosomes came from the father’s sperm– 1 of each numbered chromosome and 1 sex chromosome

• These pictures are called karyotypes and are used for finding problems with the number or shape of chromosomes and for determining sex.

Making sperm and egg• The process of making sperm or egg cells is called meiosis

• Which copy of each numbered chromosome and the sex chromosome goes into each egg or sperm during meiosis is random

– For example: your father could give the chromosome number 7 he got from his mother or the chromosome number 7 he got from his father

• Whichever chromosome gets donated, you get all the versions of the genes (alleles) on that chromosome

Sorting it out…• You get 2 copies of each chromosome

– 1 from mom and 1 from dad

• Each chromosome has genes on it

• There are 2 basic types of genes– Dominant = makes a working protein– Recessive = makes a non-working

protein

• This means there are 3 basic combinations of genes you could get from your parents for each trait– Both dominant

• This is called homozygous dominant– One dominant & one recessive

• This is called heterozygous– Both recessive

• This is called homozygous recessive

Genotypes and Phenotypes• The combination of genes you get from the chromosomes your parents

give you is called your genotype

– Dominant genes are written as capital letters• Different letters are used to represent different genes• Usually the first letter of the dominant trait is used (not always)

– Recessive genes are written as lower case letters

– Example: for the height of a pea plant,• T = dominant gene, which makes a protein that makes plants tall• t = recessive gene, which makes a protein that makes plants short• TT = homozygous dominant genotype• Tt = heterozygous genotype• tt = homozygous recessive genotype

• The actual traits you get are called your phenotype– Tall plant = dominant phenotype– Short plant = recessive phenotype

Using genotypes to figure out phenotypes

• If an organism is homozygous dominant:– They got 2 copies of the dominant gene– They make 2 working proteins– They get the dominant phenotype– Example: TT = tall plant

• If an organism is heterozygous:– They got 1 copy of the dominant gene and 1

copy of the recessive gene– They make 1 working protein and 1 non-

working protein– They get the dominant phenotype– Example: Tt = tall plant

• If an organism is homozygous recessive:– They got 2 copies of the recessive gene– They make 2 non-working proteins– They get the recessive phenotype– Example: tt = short plant

Pedigrees

• Divorces are indicated by putting a diagonal slash through the horizontal line between a husband and wife

• A dead person is indicated putting a diagonal slash through the circle or square

• Diamond shapes are used if the person’s gender is not known

• Carriers will not always be marked. Sometimes a fully colored in symbol will actually be a carrier.

Tracking traits on pedigrees• R = dominant gene = can roll tongue

• r = recessive gene = cannot roll tongue

• Possible genotypes & phenotypes:

• RR = homozygous dominant = can roll tongue

• Rr = heterozygous = can roll tongue

• rr = homozygous recessive = cannot roll tongue

TIPS:

• Fill in the known genotypes first.

• Look at the kids to figure out what the parents might be for the unknowns.

Mendel’s Experiments• Gregor Mendel was an Austrian monk whose experiments with pea plants helped us

understand how traits are inherited

• He crossed purebred plants with similar and different traits to see what traits the offspring received

– We now know that purebred means homozygous (carrier means heterozygous)– Mendel did not know anything about DNA, genes or genotypes when he was doing his work

• He then crossed the offspring with each other to see what traits they would pass on– Plant brothers and sisters are allowed to make offspring

• In the bottom row of crosses, the short trait “skipped a generation”. This is caused by recessive genes.

Punnett Squares• To figure out what traits the offspring will get, we have to

account for all the possible combinations of sperm and egg that could fertilize each other– We can do this with a tool called a Punnett Square– Punnett Squares do NOT show you exactly what will happen

• They show the possible genotypes and phenotypes of the offspring• They can show what genotypes and phenotypes are not possible in the

offspring• They can tell you the probability of each of genotype and phenotype

happening

Steps for filling in Punnett Squares

1. Figure out what letter is being used to represent the gene (see chart from earlier in notes)

2. Figure out what the dominant trait is and what the recessive trait is

3. Figure out the parent genotypes

4. Draw the Punnett Square

5. Fill in the parent genotypes

6. Fill in the possible offspring genotypes

7. Answer the question being asked

Let’s cross a pea plant heterozygous for its yellow peas with a plant homozygous recessive for its green peas

Probability• Probability is the likelihood, odds, or chance

of an event happening– This can be represented as a fraction,

decimal, or a percent

• In order to calculate the probability of a certain type of offspring happening from a set of parents:– Count the number of offspring that have the

genotype or phenotype you’re looking for• Put this number in the top (numerator) of a

fraction– Count the number of total offspring in the

Punnett square• Put this number in the bottom (denominator) of

a fraction

• To figure out the probability of multiple events happening, multiply their probabilities together

Explaining Mendel’s Results

• Let’s cross the purebred tall plant with the purebred short plant

• Results = ______________

• Let’s cross the offspring from the first Punnett square with each other

• Results = ______________

Polygenic Traits• Polygenic traits are characteristics of organisms that are

controlled by more than one gene– This term is also (sometimes) applied to genetics problems where

people are studying more than one trait at a time

• We can still use Punnett Squares to see how these might be passed on– There are 2 methods:

• One method is a little longer, but the math is easier• The other method is faster, but requires more math

• Let’s solve the same problem both ways and see which you prefer:– What is the probability of getting a plant with purple, terminal flowers

from a cross between two parents that are both heterozygous for their purple, axial flowers?

More boxes, less math method

1. Figure out what letter is being used to represent the gene

2. Figure out what the dominant trait is and what the recessive trait is

3. Figure out the parent genotypes

4. Draw the Punnett Square

5. Fill in the parent genotypes

6. Fill in the possible offspring genotypes

7. Answer the question being asked

What is the probability of getting a plant with purple, terminal flowers from a cross between two parents that are both heterozygous for their purple, axial flowers?

Fewer boxes, more math method

1. Figure out what letter is being used to represent the gene

2. Figure out what the dominant trait is and what the recessive trait is

3. Figure out the parent genotypes

4. Draw the Punnett Square

5. Fill in the parent genotypes

6. Fill in the possible offspring genotypes

7. Answer the question being asked

What is the probability of getting a plant with purple, terminal flowers from a cross between two parents that are both heterozygous for their purple, axial flowers?