Who is Gregor Mendel? Introduction to Mendelian genetics

Who is Gregor Mendel?

Introduction to Mendelian genetics

TRUE OR FALSE?

1. Girls inherit more traits from their mother than their father2. You have inherited traits that are not apparent3. Color blindness is more common in males than females4. Identical twins are ALWAYS the same sex5. A person can transmit genetic traits to their offspring which

they themselves DO NOT show6. The father determines the sex of a child7. The total number of male births exceeds female births each

year8. Acquired characteristics, like mathematical skills, can be

inherited9. Fraternal twins are more closely relates to each other than

to other siblings

Answers

1. False2. True3. True4. True5. True6. True7. True8. False9. False

Genetics

The field of Biology devoted to understanding how characteristics are passed from parents to offspring

Gregor Mendel In the 19thcentury, Mendel studied

heredity-which is the transmission of characteristics from parent to offspring Mendel is most famous for studying pea

plants He studied what he called “factors” in

pea plants Factors would be things like tall or short

(height), or yellow or green (pod color)

Some of Mendel’s Factors

Gregor Mendel First, Mendel grew true-breeding plants

According to Mendel, true-breeding plants are plants that will always produce offspring with the same traits

So a true-bred pea plant with purple flowers will only produce plants with purple flowers because it only has the “factors” for purple (not white).

Gregor Mendel: P generation

Mendel bred two opposite true-breeding plants For example, he bred a true-breeding purple

flower pea plant and a true-breeding white flower pea plant

He called this his P generation – parent generation

Gregor Mendel:F1 generation

All of the offspring of the P generation (which he called the F1 generation) turned out purple Mendel called purple flower color the

dominant factor He hypothesized that when the

dominant factor was present, the recessive factor(white color) did not show.

Gregor Mendel:F2 generation

Next, Mendel crossed the offspring from the F1 generation (he called this the F2 generation)

–He observed that about 75% of the flowers were purple and about 25% were white

–This is equal to about a 3:1 ratio

Mendel

P Generation(true-breeding parents)

F1 Generation(hybrids)

F2 Generation

Purpleflowers

White flowers

All plants hadpurple flowers

Mendel’s Real Results

Mendel’s Laws

Keep in mind that Mendel knew nothing of Punnett squares, genes, alleles, or even DNA!!! All he could do was observe

phenotypes and record ratios and other statistics

He came up with 2 important laws as a result of his observations.

Mendel’s Laws:Law of Segregation

Mendel concluded that each plant gets two factors (alleles) for a characteristic and when the plant reproduces, these two factors separate or segregate. So… Each gamete (sex cell) gives one factor

(allele) AND therefore… Each offspring gets one factor from

each parent

Law of Segregation:Punnett Squares

Alleles separate

Alleles separate

Mendel’s Laws: Law of Independent Assortment

Mendel did experiments using more than one trait (like height and seed color) He noticed that one trait did not

influence the inheritance of another trait

In other words, different factors separate independently of each other during the formation of gametes

Mendel’s Laws: Law of Independent Assortment Examples:

Pea plants can be short or tall Their seeds can be green or yellow Short plants can have green or yellow seeds Tall plants can have green or yellow seeds So the inheritance of one does not affect the

inheritance of the other. Mendel noticed this with all the traits he

studied

Independent Assortment

Mendel’s Laws

Independent Assortment is not always true-•If different genes are located on the same chromosome, then they will most likely be inherited together•These are called Linked Genes

What were Mendel’s factors in reality? We call these alleles today –Alleles are alternative forms of a gene

Alleles for flower color were purple and white

The characteristics (like height) are caused by genes on DNA

Genes are segments of DNA that code for one protein

Each gene has 2 alleles, or versions (1 from mom and one from dad)

What were Mendel’s factors in reality?The reason alleles come in pairs is because chromosomes come in pairs (homologous pairs)!!•One allele on each chromosome!•WHAT A COINCIDENCE!!!

The Genetics of Mendel’s Experiments

Some Vocab Dominant trait-masks the recessive

Shown with capital letters Recessive trait-only shows if

dominant is not present Shown with lower case letters

Phenotype-physical appearance For example purple, wrinkled, tall, etc


Some Vocab Genotype-genetic makeup

This is usually abbreviated with letters like Gg, FF, or hh

Genotypes for a trait are usually2 letters because you get 2 alleles (1 from mom and 1 from dad)

Homozygous-two of the same alleles (like HH or hh)

Heterozygous-two different alleles (like Hh)


Mendel’s P generation had the genotypes FF (for purple) and ff (for white) True breeding is also homozygous

FF is homozygous dominantff is homozygous recessive


We can show the results Mendel observed using a Punnett Square: A Punnett Square shows possible

genetic combinations in the zygotes Mendel crossed his true breeding

purple and white flower pea plants We write this as FF x ff

LET’S DO THIS ON THE BOARD


What Mendel did not know: All of F1 pea plant flowers heterozygous

(two different alleles), or Ff That is why they were all purple

Remember dominant alleles mask recessive alleles

So with one purple allele present and one white, only purple would show as it is dominant

F2 generation

LET’S EXAMINE EACH RATIO FOR EACH CROSS: F2 Generation

What genotypes do you start with? How are they crossed? What are your results? What is the genotypic ratio (genes)? What is the phenotypic ratio of purple

(F) to white (f)?

Punnett Square Examples

•Let’s do a Punnett square for BB x Bb•B= black fur in bunnies•b= white fur in bunnies•Black fur is dominant

•What is the genotypic ratio?•What is the phenotypic ratio?•What are the chances for a white bunny?

Punnett Square Examples

•Let’s look at a heterozygous cross•Bb x Bb

•What is the genotypic ratio?•What is the phenotypic ratio?•What are the chances for a white or black bunny?

Predicting the Results of Heredity

What do these ratios and percents mean? If we flip a coin, there is a 50% chance that it

will land on heads. But it is still possible to get 5 tails in a row (although it is highly UNLIKELY!)

The more times you flip it, the more likely your results will be 50:50

If Bb and Bb bunnies mate, there is a 1:4 chance the offspring will be white (this does NOT mean that they will or will not have white bunnies)

If they have LOTS of children, about 25% of them will be white

REMEMBER… Homozygous dominant means 2 BIG

letters Heterozygous means one big one

little Homozygous recessive means 2 little

letters If an organism shows the dominant trait,

then the can be either heterozygous OR homozygous dominant

Test Cross

When genotypes are not known, a test cross can be performed to figure it out The organism with an unknown genotype

is crossed with a homozygous recessive individual.

Test crosses are often used in breeding (like dog breeding) to determine is organisms are really “pure bred” (homozygous) for desired characteristics

Test Cross Problem:

Let’s say you want to breed black bunnies and you do not want any white bunnies

What would be the only parents’ genotypes to produce black bunnies?

BB x BB There are 2 ways to know for sure which

black bunnies are homozygous and which are heterozygous: expensive genetic testing, or test crosses

Test Cross

Solution: We take some black bunnies and

mate them with white bunnies (homozygous recessive)

Let’s look at the Punnett Square results to see the possible results

Remember, black bunnies can be either BB or Bb

Test Cross – Punnett Squares(try each cross)

If a BB is crossed with bb, no white bunnies are produced

If a Bb is crossed with bb, then white bunnies may be produced

If a test cross produces white bunnies, we know the unknown genotype is Bb; if not the genotype is BB

The cross would be performed multiple times to be sure of the results

Predicting Dihybrid Crosses

When 2 traits are being looked at… Let’s do a cross between two heterozygous tall, heterozygous purple flowered pea plants

So, TtFfx TtFf For each plant, we now look at genotype for color and height

Predicting Dihybrid Crosses Instead of 2 possible gametes, there will be 4

So, the Punnett Square will be 4 x 4 Phenotypic Ratios

Tall, purple : tall, white : short, purple : short, white Keep same letters together, capitals 1st You will not be asked for genotypic ratios for

dihybrid crosses What are the phenotypic ratios?

LET’S DO IT ON THE BOARD

Complex Inheritance

Mendel observed monogenic traits and no linked genes…It’s not usually that simple….

Other Types of Inheritance Incomplete Dominance

The phenotype of the heterozygote is intermediate between phenotypes of the dominant and recessive traits

Example: when a homozygous red carnation is crossed with a homozygous white carnations, then pink carnations are produced

We usually don’t use lower case letters in this type of inheritance because nothing is really dominant

Incomplete Dominance

•RR = Red•RW= pink•WW= white

Let’s look at the cross on the board

Other Types of Inheritance

Codominance Occurs when both alleles for a trait are expressed in heterozygous offspring

Codominant alleles are often symbolized with different letters

Codominance

BB = Brown BW= Roan WW= White

Notice both brown and white are present in the heterozygous genotype

Codominance

LET’S EXAMINE THE PUNNETT SQUARE ON THE BOARD

Roan x Roan BW x BW

What are the ratios for each phenotype?


Multiple Alleles: Genes with 3 or more alleles (or variations)

Human blood type shows codominance and it has multiple alleles-A, B, and O

Blood Type Human blood types have 3 alleles A, B, and O.

Each person still only gets 2 alleles, but there are 3 possibilities

O is recessive to A and B, A and B are codominant: Genotype AO or AA = A blood Genotype BO or BB = B blood Genotype OO = O blood Genotype AB = AB blood (both alleles

expressed)

Blood Type

Terminology Genotype

Heterozygous B BO Heterozygous A AO Homozygous recessive

OO Homozygous A AA Homozygous B BB AB(technically heterozygous) AB

Codominance Punnett Square

LET’S EXAMINE THE PUNNETT SQUARE ON THE BOARD

Heterozygous A with Heterozygous B AO x BO



Sex-Linked Genes and Traits Remember sex chromosomes are the

chromosomes that determine the sex of an organism

So these are traits/genes carried on sex chromosomes

These traits are symbolized using a superscript on the X or Y, such as Xr or XR

Other Types of Inheritance Sex-Linked Genes and Traits Examples:

In fruit flies, the gene for eye color is on the X chromosome. Red (XR) is dominant, white (Xr) is recessive.

To have white eyes, females must have the genotype XrXr, or in other words TWO white alleles

To have white eyes, males must have the genotype XrY, or in other words ONE white allele

This is why X chromosome sex-linked traits are more common in males

Try the Punnett Square

Homozygous red eyed female x white eyed male XRXR x XrY



Polygenic Inheritance: Traits that are controlled by more than one gene

Most human traits are polygenic

Examples are height, skin color, eye color, and hair color


Complex Characters: Characters that are influenced by genetics AND the environment

Skin color and height are examples


Sex-Influenced Traits: Traits in which males and females show

different phenotypes even though they have the same genotypes

Baldness is an example- it is dominant in men, but recessive in women

The differences are mainly due to males and females producing different hormones (chemical signals)


Single Allele Traits Traits where there is only one allele

If you have the allele you have the trait-there is no recessive

Huntington’s disease is an example

Pedigrees

Another way to show heredity….

Pedigree is a chart or “family tree” that tracks which members of a family have a particular trait.

Pedigrees In pedigrees, carriers have one copy

of the recessive allele So they CARRY the trait, but they do

not show it Pedigrees can be used to make predictions about

Future offspring The genotype of individuals in the

pedigree

Pedigrees

The first pedigree tracks the widow’s peak, so the filled in shapes have a widow’s peak Widow’s peak is a dominant trait

Carriers are not always shown on pedigrees

Think about what alleles their parents can give them

Ww

Ww

Ww

Ww Ww

ww ww

ww ww ww

wwWWorWw

First generation(grandparents)

Second generation(parents plus aunts and uncles)

Thirdgeneration(two sisters)

ww= no widow’s peakWW= widow’s peak

Dominant trait (widow’s peak)

Chromosome Mutations Chromosome mutations

involve changes in the structure of a chromosome or the loss or gain of a chromosome.

–Deletion: The loss of a piece of chromosome due to breakage

–Inversion: A chromosomal segment breaks off, flips around, and reattaches

-Missence: A change in chromosomal arrangement by insertion of DNA segment

Translocation-A piece of chromosome breaks off and reattached to a nonhomologous chromosome

Chromosome Mutations

Nondisjunction-When a chromosome fails to detach from its homologue during meiosis, so one gamete gets an extra chromosome

Instead of a haploid number (n) or diploid (2n), the gamete has 3 chromosomes (3n)

Chromosome Mutations (Examples)

Down’s syndrome Nondisjunction of chromosome 21 in the egg

cell produces 3 copies of chromosome 21 Symptoms include: heart defects, stunted

growth, mental retardation Cystic fibrosis

Can be caused by several mutations on chromosome 7 (insertion, missence)

Symptoms include: problems with respiratory and digestive systems

Chromosome Mutations (Examples)

Klinefelter’s syndrome A male receives an extra X chromosome (XXY)

because of nondisjunction of egg cell Symptoms: Boyish, rounded look (despite age)

and often infertility Trisomy 18 (Edward’s syndrome)

Nondisjunction of chromosome 18 results in 3 copies of chromosome 18

Much more severe problems than Down’s syndrome. Only 10% of births survive to their first birthday

Documents

Who is Gregor Mendel? Introduction to Mendelian genetics