Genetics and Heredity

Genetics

•Genetics is the study of heredity

• It looks at understanding the biological properties that are transmitted from parent to offspring

DNA

• DNA (deoxyribonucleic acid) is the genetic code for almost every living organism

• DNA is often called a double helix because of the way it coils

– Some ‘organisms’ like mitochondria use RNA (ribonucleic acid) instead of DNA

• There are 4 parts to DNA called nucleotides: A (adenine) T (thymine) C (cytosine) G (guanine)

• A long sequence of nucleotides makes up a gene:

– A gene is a portion of the DNA responsible for certain traits or characteristics

– Ex) body part structure, hormones, proteins, eye color, hair color etc . . .

• Each cell in our body has exactly the same DNA, but only certain genes are ‘turned on’ at a time

– Ex) the genes that determine hair color are only turned on in our hair follicles, and the genes that determine our height are only ‘turned on’ in our bone and muscle cells

• The DNA in our cells is very long and would stretch over a meter if uncoiled

• Since it is so long, it is divided into chromosomes

Chromosomes

• Chromosomes are small, rod shaped bodies in the nucleus of a cell that contain the DNA

• Humans have 23 pairs of chromosomes– There are 46 chromosomes in total –

you get 23 from your mother, and 23 from your father

DNA Coiling

Dominant and Recessive Traits

• Most of the time, our traits are controlled by more than one gene; sometimes though they are controlled by a single gene

• When a single gene controls a trait it can be dominant or recessive

• Dominant: ‘stronger’ trait

• Recessive: ‘weaker’ trait

• Ex) If you get a dominant trait from either of your parents it will override or mask a recessive trait

– Brown eyed genes are dominant over blue eyed genes

• Even though a recessive trait is not physically shown, you still carry the trait, and can pass it on to your offspring

Genotype

• A genotype is the combination of genes inherited from your parents

– B = brown eyes (dominant genes are always written with capital letters)

– b = blue eyes (recessive genes are always written with small letters)

• A person with brown eyes could have either BB or Bb as their genotype

• A person with blue eyes could only have bb as their genotype

What do you think would happen if one parent had BB brown eyes? What color

eyes would their offspring have?

• Homozygous: if both alleles (individual genes) are the same– Ex) BB or bb

• Heterozygous: if the alleles are different– Ex) Bb

Phenotype

• The phenotype is the visible expression of genes:

– Ex) The person has brown eyes, or the person has blue eyes

• The color is the phenotype in this example

Some traits controlled by a single gene

• Tongue Rolling• Widow's Peak• L/R interlocking

finger • Attached earlobes• Hitchhiker Thumb

(r) • Chin fissure • Darwin tubercle• Polydactyl • Dimples • Long eyelashes • Short big toe

Punnett’s Square

• A Punnett’s Square is a table used to figure out the probability of offspring having a certain trait

• They allow us to easily cross the genes of the mother and father to determine the possible genotypes and phenotypes

• A Punnett’s Square takes the alleles from each parent and examines the outcomes

• The father’s genes usually go on the top and the mother’s genes go on the side

Genotypes:• 25 % will be homozygous dominant (BB)• 50 % will be heterozygous dominant (Bb)• 25 % will be homozygous recessive (bb)

Phenotypes:• 75% will be brown eyed• 25% will be blue eyed

• Example 1: Curly hair (C) is dominant over straight hair (c). If a homozygous recessive man (cc) and a heterozygous dominant (Cc) woman have a child, what will the genotypes and phenotypes be? Use a Punnett’s Square to find the answer.

c c

C

c

Step 1: Fill in the alleles for the mother and father

c c

C Cc Cc

c cc cc

Step 2: Cross the alleles to fill in the table (the dominant gene is always written first)

Step 3: from the table, figure out the genotypes and phenotypes

Genotypes:• 50% heterozygous dominant• 50% homozygous recessive

Phenotypes:• 50% curly hair• 50% straight hair

c c

C Cc Cc

c cc cc

Try This . . .

• Example 2: Draw the Punnett’s square for a heterozygous red corsage (Rr) matched with a heterozygous red corsage (Rr). Give the genotypes and phenotypes.

R r

R RR Rr

r Rr rr

Genotypes:• 25% homozygous dominant• 50% heterozygous dominant• 25% homozygous recessive

Phenotypes:• 75% red corsages• 25% white corsages

• Example 3: Your father is homozygous dominant for tongue rolling, while your mother is heterozygous dominant for tongue rolling. Using a Punnett’s Square to determine the genotypes and phenotypes.

T T

T TT TT

t Tt Tt

Genotypes:• 50% homozygous dominant• 50% heterozygous dominant

Phenotype:• 100% tongue rolling

Incomplete Dominance

• Some traits are controlled by more than one gene

• When there isn’t a dominant trait it is called incomplete dominance

• The most common examples are with plants:

– Red flowers and white flowers are mixed, but the result is a pink flower, or a red and white striped flower; neither color dominates

Sex-linked Inheritance

• X and Y chromosomes determine sex (gender)

• Females: XX Males: XY

• The X and Y chromosomes also carry the genes for certain characteristics including hormone production and development of sex organs

• The X chromosome carries more traits than the Y as it is larger

• Sex linked traits tend to impact men more than woman because the men only get one X chromosome

Sex linked Examples

• Colour blindness is carried on the X. A person needs to have all of their X chromosomes affected to have the disorder

– This means that in order for a female to have the trait, both her X chromosomes need to have the gene

• Hemophelia is an X-linked recessive trait. Your blood does not clot properly.

Xh Y

XH XHXh

CarrierXHY

Affected

Xh XhXh

UnaffectedXhY

Unaffected

Karyotyping

• Karyotyping is a process where we photograph chromosomes of a cell

• Chromosomes are studied and paired up and analysis is done to identify chromosomal abnormalities

• This is usually done before a baby is born

Diseases and Defects

• There are at least 4000 hereditary diseases

• Examples include muscular dystrophy (cannot control muscles), Huntington’s chorea (attacks nerve cells in the brain), and cystic fibrosis (affects the pancreas and the bronchioles in the lungs)

Try this . . .

• Example 4: If a homozygous dominant woman and a colour blind man have kids, what percentage will be colour blind?