What is Genetics?Genetics is the scientific study of

heredity

Genomes

A genome is the full set of genetic information that an organism carries in its DNA.

The analysis of any genome starts with chromosomes.

A human karyotype

Karyotypes

A karyotype shows the complete diploid set of chromosomes grouped together in pairs, arranged in order of decreasing size.

Autosomal Chromosomes

The other 22 pairs of non-sex chromosomes are called autosomal or autosomes.

Male = 46, XY

Female = 46, XX

Transmission of Human Traits

Many human traits follow a pattern of simple dominance.

For example, the gene MC1R helps determine skin and hair color.

Some of MC1R’s recessive alleles produce red hair.

An individual with red hair usually has two of these recessive alleles.

Transmission of Human Traits

The X and Y chromosomes determine sex.

The genes located on them show sex linkage.

X-Chromosome Inactivation

Most genes in one of the X chromosomes are switched off, forming a region known as a Barr body.

Female calico cats are

tri-colored.

Male cats can have spots

of only one color.

The color of spots on

their fur is controlled by a

gene on the X

chromosome.

Gregor Mendel monastery priest (1st important studies of inheritance)

Heredity – the passing on of characteristics from

parents to offspring

Genetics is the branch of biology that studies the

patterns of inheritance and variations in organisms

Mendel was the first to succeed in predicting how traits

would transfer from one generation to the next.

Mendel’s PEAs!!! https://www.youtube.com/watch?v=Mehz7tCxjSE

Rule of Dominance: if you have 2 different alleles

and 1 is dominant, that is what trait will be

expressed.

• If the F1 generation plants (second generation) had a tall allele from one parent and a short allele from the other, they all appeared tall.

• Therefore, one trait was dominant over the other trait (recessive).

• Plants with two alleles for tallness (TT) were tall.

• Plants with two alleles for shortness were short (tt)

• Plants with one allele for tallness and one for short (Tt) were tall

Law of Segregation: only 1 copy of a

gene is found in a gamete• If an organism has two different alleles (genes) for a trait,

that organism can make two different types of gametes.

– Tt plant can produce T gametes and t gametes

• Fertilization from a Tt + Tt cross will result in random

pairs of the available gametes

(four possible combinations)

– T from male + T from female = TT

– T from male + t from female = Tt

– t from male + T from female = Tt

– t from male + t from female = tt

YES – this is related to Punnett Squares!!!

Sex Chromosomes

Two of the chromosomes in the human genome are sex chromosomes.

Females: two X chromosomes

Males: one X chromosome; one Y chromosome

Dihybrid Punnett Square Put one of each type of possible gamete combination from one

parent on top of the square

Put one of each type of possible gamete combination from the other parent on the side of the square

Fill each box with the gamete of that box’s row and column

The possible offspring combinations can be seen

The ratio of offspring phenotypes after a heterozygous dihybrid cross is 9:3:3:1 In this case we see 9 round yellow,

3 round green, 3 wrinkled yellow and 1 wrinkled green

What is a Trait?A trait is a specific characteristic that varies

from one individual to another.

Examples: Brown hair, blue eyes, tall, curly

What is an Allele?Alleles are the different

possibilities for a given trait.

Every trait has at least two alleles (one from the mother and one from the father)

What are Genes?Genes are the

sequence of DNA that codes for a protein and thus determines a trait.

Dominant vs. Recessive

Dominant - Masks the other trait; the trait that shows if present

Represented by a capital letter

Recessive – An organism with a recessive allele for a particular trait will only exhibit that trait when the dominant allele is not present; Will only show if both alleles are present

Represented by a lower case letter

R

r

Dominant & Recessive Practice

TT - Represent offspring with straight hair

Tt - Represent offspring with straight hair

tt - Represents offspring with curly hair

T – straight hair

t - curly hair

Genotype vs. Phenotype Genotype – The genetic makeup of an organism;

The gene (or allele) combination an organism has.

Example: Tt, ss, GG, Ww

Phenotype – The physical characteristics of an organism; The way an organism looks

Example: Curly hair, straight hair, blue eyes, tall, green

Homozygous vs. Heterozygous

Homozygous – Term used to refer to an organism that has two identical alleles for a particular trait (TT or tt)

Think homo=same

Heterozygous - Term used to refer to an organism that has two different alleles for the same trait (Tt)

RR

Rr

rr

Punnett SquaresPunnett Square – Diagram showing the

gene combinations that might result from a genetic cross

Used to calculate the probability of inheriting a particular trait

Probability – The chancethat a given event will occur

Punnett Square

Parent

Parent Offspring

How to Complete a Punnett Square

Y-Yellow

y-white

Genotype:

1:2:1

(YY:Yy:yy)

Phenotype:

3 Yellow

1 White

You Try It Now! Give the genotype and phenotype for the following

cross: TT x tt (T = Tall and t = Short)

TT x ttStep One: Set Up Punnett Square (put one parent on the top

and the other along the side)

T Tt

t

TT x ttStep Two: Complete the Punnett Square

T Tt

t

Tt Tt

Tt Tt

TT x ttStep Three: Write the genotype and phenotype

T Tt

t

Tt Tt

Tt Tt

Genotype:

4 - Tt

Phenotype:

100% Tall

Remember: Each box is 25%

You Try It Now! Give the genotype and phenotype for the following

cross: Tt x tt

Tt x ttStep One: Set Up Punnett Square (put one parent on the top

and the other along the side)

T tt

t

Tt x ttStep Two: Complete the Punnett Square

T tt

t

Tt tt

Tt tt

Tt x ttStep Two: Complete the Punnett Square

T tt

t

Tt tt

Tt tt

Genotype:

Tt - 2 (50%)

tt - 2 (50%)

Phenotype:

50% Tall

50% Short

Remember: Each box is 25%

Incomplete Dominance Incomplete Dominance - Situation in

which one allele is not completely dominant over another.

Example – Red and white flowers are crossed and pink flowers are produced.

Codominance Codominance - Situation in which both

alleles of a gene contribute to the phenotype of the organism.

Example – A solid white cow is crossed with a solid brown cow and the resulting offspring are spotted brown and white (called roan).

+

Blood Type Human blood types demonstrate multiple alleles (more

than two alleles of the gene) A, B, O

Human blood types also demonstrate codominance –where heterozygous alleles can be expressed equally A and B are codominant

O is recessive

These alleles are written as IA, IB, and i IAIA or IAi will have type A blood

IBIB or IBi will have type B blood

IAIB will have type AB blood

ii will have type O blood

Blood Type Crosses

Type O mother and Type AB father

IA

i IAi

50% blood type A

50% blood type B

100% type AB

i IBi

IBi

IAi

IB

IA IAIB

IA

Homozygous Type A mother and

Homozygous Type B father

IB IB

IAIB

IAIBIAIB

Sex Chromosomes

More than 1200 genes are found on the human X chromosome.

The human Y chromosome contains only about 140 genes.

Sex-Linked Traits• Traits controlled by genes carried on the X or Y

chromosomes are called sex-linked traits

• Most of these types of traits are carried on the X chromosome

• The alleles for different forms of the sex-linked traits are shown as superscripts on the X

– XRXR XRXr XrXr

– XRY XrY

• Because the Y does not carry an allele, a male could not be heterozygous for a sex-linked trait

Sex-Linked Crosses

Colorblind mother and Normal father

XN Y

Xn XNXn

Normal daughters (carriers)

Colorblind sons

50% Normal daughters and sons

50% colorblind daughters and sons

Colorblindness is a recessive sex-linked trait. Use XN for the normal allele and

Xn for the colorblind alleleHeterozygous Normal mother

and Colorblind father

Xn XnY

XnY

XNXn

Xn Y

XN XNXn

Xn XnY

XNY

XnXn

Examples of Multiple Alleles

1. Blood Type – 3 alleles exist (IA, IB, and i), which results in four different possible blood types

2. Hair Color – Too many alleles exist to count

There are over 20 different shades of hair color.

Multiple Alleles There Are Always Multiple Alleles!

Genetic inheritance is often presented with straightforward examples involving only two alleles with clear-cut dominance. This makes inheritance patterns easy to see.

But very few traits actually only have two alleles with clear-cut dominance. As we learn more about genetics, we have found that there are often hundreds of alleles for any particular gene.

We probably know this already - as we look around at other people, we see infinite variation.

Polygenic Trait Polygenic Trait - Trait

controlled by two or more genes.

Polygenic traits often show a wide range of phenotypes.

Example: The wide range of skin color in humans comes about partly because more than four different genes probably control this trait.