Welcome to genetics!

What is genetics?

The scientific study of heredity - the genetic properties or inherited features of an organism

•Molecular genetics: study of structure and function of DNA and RNA

•Population genetics: study of genes within populations, including gene frequency, the gene pool and evolution

Like Begets Like

List differences in physical

characteristics of you and your

mother

Pre-Mendel Belief of inheritance:This theory stated that offspring would have traits

intermediate between those of the parents (Blending Concept)

Mendel’s Law of Segregation:Each organism contains two factors for each trait, and the factors segregate during the formation of gametes so that each gamete contains only one

factor for each trait

Mendel in his

gardenMendel observed over 7,324 peas for one test cross

The history of genetics…

• Meet Gregor Mendel...

• Timeline of genetics I

Garden Pea Anatomy and

Traits

Easy to grow, short generation time, could control pollination, wide variety of traits

Mendel looked at 7 major traits

Plant Height (tall, short)

Pod color (green, yellow)

Pod Shape (Inflated, constricted)

Seed Shape (round, wrinkled)

Seed Color (yellow, green)

Flower color (white, purple)

Flower position (side, top)

What Mendel didn’t know, was that genetic information

is found on our chromosomes

© 2007 Paul Billiet ODWS

Let’s learn about the relationship between chromosomes and genetics

• http://www.youtube.com/watch?v=eOvMNOMRRm8

CHROMOSOMES

© The Human Genome Project: Biocomputing Admin Ed Yung

Eukaryotic Chromosomes

• Found in the nucleus• Condensed and visible during cell division• At the beginning of mitosis they can be seen to

consist of two threads (sister chromatids) joined by a centromere

• The sister chromatids are identical copies• During mitosis the sister chromatids separate

and are placed into two nuclei

© 2007 Paul Billiet ODWSImage believed to be in the Public Domain

Numbers of chromosomes

• Constant for each cell in the body (except sex cells which only have half sets).

• Constant throughout the life of an individual (you don’t lose or gain chromosomes)

• Constant for all members of a species

© 2007 Paul Billiet ODWS

Human chromosomes

Image believed to be in the Public Domain

The chromosomes of a human female

© 2007 Paul Billiet ODWS Image believed to be in the Public Domain

Down’s syndrome

Image believed to be in the Public Domain

MouseMaize

© A. Lane RayburnImage believed to be in the Public Domain

Organism Chromosome numbers

Human 46

Chimpanzee 48

House Mouse 40

Maize 20

Identifying chromosomesChromosomes can be identified by:• Their size• Their shape (the position of the

centromere) NB Chromosomes are flexible

• Banding patterns produced by specific stains (Giemsa)

Chromosomes are analysed by organising them into a KARYOTYPE

© 2007 Paul Billiet ODWS

© Biologyreference.com

Essential vocabulary of genetics:

• Gene: A length of DNA that is the unit of heredity

• Genotype: The genetic make-up of an organism, in terms of the allels present

• Allele: Any of two or more alternative forms of a gene

• Phenotype (trait): Physical or other features of an organism. Caused by combination of genotype AND environment

Homologous chromosomes:

Alleles

Each cell has (at least) 2 genes for any characteristic

Each cell has (at least) 2 genes for any characteristic

Homozygous Homologous chromosomes have identical alleles of a particular gene. Two identical homozygous individuals that breed together will always have the same characteristics

‘Pure breeding’

Heterozygous: Chromosomes have two different alleles of a particular gene (Ff).•‘not ‘’pure breeding’…

Dominant and recessive alleles

• A dominant allele (e.g. F) will be expressed if it is present

• A recessive allele (e.g. f) will only be expressed when there is no dominant allele of the gene present

Dominant and recessive genes

A dominant gene (allele) is one that will be expressed physically if it is dominant

A recessive allele (gene) is one that will only be expressed when there is no dominant allele of the gene present (e.g. t or g)

Cystic fibrosis is disease associated with expression of a recessive gene

Rules of ‘Mendelian’ Inheritance

1. Inherited traits are determined by genes that are passed from parents to children (Mendel’s Theory of particulate inheritance).

2. A child inherits two sets of genes—one from each parent.

3. A trait may not be observable, but its gene can be passed to the next generation

Mendelian Inheritance

Each person has 2 copies of every gene—one copy from mom and a second copy from dad. These copies may come in different variations, known as alleles, that express different traits.

For example, 2 alleles in the gene for freckles are inherited from mum and dad:

– allele from mum = has freckles (F)– allele from dad = no freckles (f)– child has the inherited gene pair of alleles, Ff

(F allele from mom and f allele from dad).

Genes, dominance and hybrids

Mendel bred peas with contrasting physical characteristics and compared their offspring.

•Each original pair of chromosomes are termed the P(parental) generation.

•The offspring are called the F1 (filial) generation (filius/ filia – son/daughter)

•Offspring of parents with different traits are called hybrids (F1 hybrids)

Testing inheritance: The F1 cross…

• To test for recessive genes, Mendel allowed all 7 kinds of F1 hybrid plants to produce a second (F2, second filial) generation by self-pollination

• He crossed the F1 generation with itself to produce the F2 offspring

• Roughly 25% of the F2 plants demonstrated the recessive allele!

What type of cross did Mendel perform when he needed to determine the geneotype of an organism?

Definitions

Autosome: any chromosome that is NOT a sex

chromosome

Homework for Tuesday…

Play the Dragon Game...

Research about a type of genetically inherited disease:

Find out about a type of inherited disease

Genetic Diseases

Autosomal Dominant Disorders

Neurofibromatosis

Huntington’s Disease

Autosomal Recessive Disorders

Tay Sachs

Cystic Fibrosis

Inherited diseases

• Phenylketonuria

• Sickle Cell disease

• Huntingdon’s disease

• Cystic fibrosis

Inherited diseases

Simple inheritance

• ‘Single gene inheritance’

• ‘mono-hybrid cross’• How Mendel worked

(1)• How Mendel worked

(2)

Mendel’s WorkAll of these produce approximately a 3:1 ratio.

Law of Probability

What are the chances of tossing two heads in a row?

Five heads in a row?

How about having two red heads in a row?

Testing inheritance: The F1 cross…

• To test for recessive genes, Mendel allowed all 7 kinds of F1 hybrid plants to produce a second (F2, second filial) generation by self-pollination

• He crossed the F1 generation with itself to produce the F2 offspring

• Roughly 25% of the F2 plants demonstrated the recessive allele!

Monohybrid cross

One-Trait Inheritance

Working out inheritance: The Punnett Square…

Let’s meet Reginald Punnett…

The rules of genetic inheritance...

Probability - likeliness that something

will or will not happenHey, Keith, how's

the weather?Weather forecast (90% chance of rain)

Winning lottery (1 in 2 million chance)

Well, there isa 50% chance

of a snow.

Probability is used in genetics to predict what offspring will look like when 2

parents are crossed.

XYou can figure

out the probabilityof getting a liger or

a tigon.

A tool that can help you understand the laws of probability and

how it is related to genetics is called a Punnett square, which is a chart that shows all the possible combinations of alleles that can result from a genetic cross.

Tt

Steps in Making a Punnett Square:

1. Draw a square and divide it into four.2. The alleles for one parent go on the top. The alleles for the other parent go down the side.3. The boxes are like a multiplication problem, with one allele contributed by each parent.

I can only give 1/2 of my DNA to my offspring, so I

will either give my baby planta T OR a t.

One Trait Inheritance

• T = tall, t = short• Complete the punnet

square in your notes.• Label the P gametes• Label the F1

generation• What is the

phenotypic ratio?• What is the

geneotypic ratio?

One Trait Inheritance

TT Tt

Tt tt

P gametes

F1 generation

Phenotypic Ratio = 3:1

Genotypic Ratio = 1:2:1

Punnett squares are written like multiplication tables.

X 1 2 3 4

1

2

3

4

Cystic fibrosis Punnet Squares (1)

Cystic fibrosis Punnet Squares (2)

Practice Questions on simple inheritance

1.

2. Jack and Jill are having a baby. What is the probability that their baby will be a BOY (XY)? What is the probability that their baby will be a GIRL (XX)?

probability of havinga baby girl (XX):__________________

probability of having a baby boy (XY):___________________

Jill - XXJack - XY

3. Use the Cats below to create a Punnett Square.Black = DominantBlue/Grey = Recessive

Parent One Genotype ________________Parent Two Genotype ________________What % is Hybrid? __________________What % Homozygous? _______________What % are Black? ___________________What % are Blue/Grey? _______________

D D

d

d

Dd Dd

Dd Dd

4.

5.

Going beyond Mendel

1. Test crosses

2. Multiple alleles – dihybrid and trihybrid crosses

3. Codominance

4. Incomplete dominance

5. Sex linkage

Beyond Mendel

• Incomplete dominance

• Codominance

• Polygenic traits

• Multiple allelic traits

• Genetic disorders

• Testing for genetic disorders

Working out inheritance

What type of cross did Mendel perform when he needed to determine the genotype of an

organism?

Further genetics

What happens if a number (say 2 or 3) traits are mixed?

Di-hybrid and tri-hybrid crosses

Mendel’s Law of independent assortment

Multiplication rule for inheritance

Inherited traits

• This trait is due to a single gene.

• Unattached earlobes are dominant

• Utah inherited traits

Inherited traits: Dimples

• Single gene

• Dimples dominant

• No dimples recessive

Inherited traits: Freckles

• Freckles dominant• No freckles recessive

What is the pattern of inheritance of multiple traits?

Dihybrid Cross (Two-factor cross)

• These involve TWO genes which control two different charaxteristics

• Unlinked genes are found on different chromosomes

• (autosomal) are those chromosomes which are not gender chromosomes

• Dihybrind cross

Mendel’s Law of Assortment

Each pair ofalleles segregatesindependently.

2. A Dihybrid Cross

Mendel’s Law of independent assortment

• The principal of independent assortment states that genes for different traits can

segregate independently during the formation of gametes (sperm and eggs).

• Independent assortment helps to account for the many genetic variations observed in

plants, animals and other organisms

What about testing multiple genotypes?

• Multiplication Rule: multiply the probability of one event with the probability of another event.

Ex: coin toss heads = ½

coin toss tails = ½

½ x ½ = ¼

Multiplication Rule

• What is the probability of a cross between a YyRr and a YyRr producing a YYRR?

• The probability of YY = ¼

• The probability of RR = ¼

Y

y

Y y

YY YyYy yy

R

r

R r

RR RrRr rrTherefore ¼ x ¼ =

1/16

Using the multiplication rule for a trihybrid cross

You have freckles, dimples, and a unattached earlobes. Your S.O. has freckles and dimples, and attached earlobes. In other words,

 FfDdUu x  FfDduuQuestion: What is the chance your darling child would have all three recessive phenotypes: no freckles (ff), no dimples (dd) or attached earlobes?

Beyond Mendel

• Incomplete dominance

• Codominance

• Multiple alleles

Beyond Mendel: Incomplete Dominance

• Sometimes, 3 different phenotypes can exist!

• In incomplete dominance, neither of 2 alleles is dominant (or recessive)

• They DO ‘blend’ to produce a new phenotype

Example: Incomplete dominance

Genotype Ratio:

•25% R1R1

•25% R2R2

•50% R1R2

Phenotype Ratio•25% Red•25% White•50% Pink

Codominance

• Both alleles are equally dominant!

• Both alleles are expressed fully, resulting in a new phenotype!

• e.g. Roan horses/cattle – red and white hairs are intermingled

Multiple alleles

Multiple alleles

• When Mendel studied pea ‘traits’, there were only 2 alleles for each of the traits he studied: (round/wrinkled, tall/short, yellow/green)

• In fact, you CAN have > 2 alleles for any trait• Human blood types are an example of a trait

with multiple alleles (A, B, O)

Human Blood types

• Type O is recessive• Types A and B are

codominant • Both Types A and B

are dominant to Type O

What does ‘blood type’ mean?

We have both antigens (markers) and antibodies in our blood

• 45% are Type O• 42% Type A

• 10% are type B• 3% are type AB

What happens if you receive the wrong type of blood in a transfusion?

Blood type inheritance (Practice)

Blood type inheritance (Practice)

• A woman who is Blood type A (homozygous) marries a man who is blood type O. What are the potential blod types of their children?

• Genotype?

• Phenotype?

High blood pressure is a polygenic trait.

• What genes could combine to contribute to high blood pressure?

High blood pressure is a polygenic trait.

• The phenotype is an interaction between a person's weight (one or more obesity genes), cholesterol level (one or more genes controlling metabolism), kidney function (salt transporter genes), smoking (a tendency to addiction), and probably lots of others too. Each of the contributing genes can also have multiple alleles.

Genetic Testing and Counseling

• Diagnosis of carriers – Biochemical (i.e. Tay-Sachs) – Genetic (i.e. Huntington’s disease) – Ethical considerations

• Fetal diagnosis- – Amniocentesis – Chorionic villi sampling

Chorionic villi sampling Chorionic villus sampling (CVS) is the removal of a small piece of tissue (chorionic villi) from the uterus during early pregnancy to screen the baby for genetic defects.

A son with cystic fibrosis (autosomal recessive) is born to a couple who appear to be normal. What are the chances that any child born to this couple will have cystic fibrosis?

And the Answer is…..

25%

In humans the allele for short fingers is dominant over that for long fingers. If a person with short fingers who had one parent with long fingers reproduces with a person having long fingers, what are the chances of each child having short fingers?

And the Answer is…..

50%

Homework: Complete the remain questions on the genetics problem set and do your genetics mini project.

Combining multiplication and addition rules to solve complex problems

• An organism with the genotype BbDD is mated with one with the genotype BBDd. Assuming independent assortment of these two genes, what is the probability that you will get a BBDD offspring?

• ¼ BBDD

More Incomplete Dominance

Sickle Cell

Pedigree Tables (1)

• A ‘pedigree chart’ is a diagram of family relaitonships that can be used to determine the mode of inheritance of a trait

• Introduction to pedigree tables

Pedigree Tables (2)

• Male = square• Female = circle• Affected = dark• Non-affected = light• Carrier = half dark/

half light

Dd Dd D D

A summary of Mendel’s Principles (1)

1. The inheritance of biological characteristics is determined by individual units – genes

(alleles). Genes are passed from parents to their offspring

2. In cases where there are 2 or more forms of the gene for a single trait, some forms of the gene may be dominant and others may be

recessive

A summary of Mendel’s Laws of inheritance (2)

3. In most sexually reproducing organisms, each adult has 2 copies of each gene – one from each parent. These genes are

segregated from each other during meiosis

4. The alleles for different genes usually segregate independently

Taking Mendelian Genetics one stage further

• The two factor cross

• Mendel’s Law of Independent Assortment

• Multiplication rule

• Addition rule

• Pedigree charts