UNIT 4: GENETICS AND INHERITANCE

Campbell & Reece: Chapters 14 and 15

1. WHAT IS GENETICS• Genetics: The study of heredity.• Heredity is the relations between

successive generations.• Why do children look a little bit like their

parents but also different?• What is responsible for these

similarities and differences?

2. MENDEL’S GENETICS• Gregory Mendel is the father of

Genetics.• Mendel discovered the basic principles

of heredity by breeding garden peas in carefully planned experiments.

• Advantages of pea plants for genetic study: Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another.

• Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another.

• He also used varieties that were true-breeding (organisms with only one variety of a type e.g. red flowers can only produce red flowers)

• In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization

• The true-breeding parents are the P generation.

• The hybrid offspring of the P generation are called the F1 generation

• When F1 individuals self-pollinate, the F2 generation is produced

• When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F1 hybrids were purple

• When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white

• Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation.

• Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids.

• Mendel called the purple flower color a dominant trait and the white flower color a recessive trait

• What Mendel called a “heritable factor” is what we now call a gene

• He did 7 other crosses using different traits and found the same phenomenon.

• Mendel noted that the gene for flower color for example exists in two versions, one for purple flowers and the other for white flowers

• These alternative versions of a gene are now called alleles

• Each gene is found at a specific locus (position) on a specific chromosome.

• The two alleles at a locus on a homologous chromosome pair may be identical, as in the true-breeding plants – they are then said to be homozygous for that trait/gene.

• Alternatively, the two alleles at a locus may differ – they are said to be heterozygous for that gene/trait.

• If the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance (we refer to it as its phenotype), and the other (the recessive allele) has no noticeable effect on appearance.

• Mendel then formulated the law of segregation, states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes

• Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism.

• An organism traits are indicated via its genotype and phenotype.

Genotype: The genetic composition of the gene, indicated by letters e.g. GG, Gg, gg. (A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele)

Phenotype: The external appearance of the gene e.g. Brown hair, white hair.

3. GENETIC CROSSES

• HOW CAN WE NOW MORE OF LESS DETERMINE WHAT WILL BE THE OUTCOME IF 2 ORGANISMS HAVE A BABY?

TWO TYPES OF GENETIC CROSSES

• MONOHYBRID CROSSES: A cross between 2 organisms where we only look an one pair of contrasting traits.

• DIHYBRID CROSS: A cross between 2 organisms where we look at two pairs of contrasting traits at the same time.

MONOHYBRID CROSS -EXAMPLE

• Determine the outcome/ F1 generation of a cross between a homozygous tall plant and a homozygous short plant. Tall plants are dominant over short plants.

STEPS TO SOLVE A CROSS PROBLEM1. What trait are we looking at?2. Choose a letter to represent the trait.3. See if you can identify which trait is dominant –

allocate the capital letter to that trait.4. Identify the recessive trait and allocate a lower case

letter to that trait.5. Determine the genotypes of the parents. –

Homozygous dominant – Two capital letters e.g. GG Homozygous recessive – Two lower case letter. E.g. ggHeterozygous – One capital letter and one lower case letter e.g. Gg

6. Start with cross

SOLUTION1. Trait – Size of plant. 2. Letter chosen to represent size of plant = T/t3. Tall plants are dominant. (Given in problem) –

Given the – “T” (capital T)4. Short plants are recessive – given the “t”

(lower case t)5. One parent is homozygous tall – TT other parent is homozygous short - tt

CROSS SHOWN AS A GENETIC DIAGRAM

Why?Tall is dominant over short plants – Babies have both alleles: tall and short

Cross shown as a punnet square

CROSS BETWEEN F1 GENERATION INDIVIDUALS (INTERBREED F1 GENERATION)

MONOHYBRID CROSS –EXAMPLE 2

A heterozygous blue eyed rabbit is crossed with a rabbit with pink eyes. What is the possibility of the babies being born with pink eyes?

SOLUTION

1. Trait: eye colour of rabbit.2. Letter used: E/e3. Dominant trait: Blue eyes (Why? The first rabbit is

heterozygous – both alleles – but blue is being expressed in rabbit eyes.) = E

4. Recessive trait: pink eyes = e5. Rabbit one – heterozygous: Ee

Rabbit two – homozygous: ee (why?)The only way that a rabbit can have pink eyes

expressed externally is if both alleles code for pink eyes.

EXAMPLE OF A DIHYBRID CROSS

Determine the F2 generation of a cross between yellow round seeded peas and wrinkled green seeded peas. Yellow and round seeds are dominant.

• Using a dihybrid cross, Mendel developed the law of independent assortment

• The law of independent assortment states that each pair of alleles segregates independently from another pair of alleles during gamete formation.

• Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes

• Genes located near each other on the same chromosome tend to be inherited together.

Dihybrid cross • In humans there is a disease called Phenylketonuria (PKU)

which is caused by a recessive allele. People with this allele have a defective enzyme and cannot break down the amino acid phenylalanine. This disease can result in mental retardation or death. Let “E” represent the normal enzyme. Also in humans in a condition called galactose intolerance or galactosemia, which is also caused by a recessive allele. Let “G” represent the normal allele for galactose digestion. In both diseases, normal dominates over recessive.

• If two adults were heterozygous for both traits, what are the chances of having a child that is completely normal?

• Has just PKU? • Has just galactosemia? • Has both diseases?

EG Eg eG egEG EEGG EEGg EeGG EeGg

Eg EEGg EEgg EeGg Eegg

eG EeGG EeGg eeGG eeGg

eg EeGg Eegg eeGg eegg

P1 EeGg x EeGg

Meiosis

F1 EG Eg eG eg EG Eg eG eg

4. DEGREES OF DOMINANCE•Complete dominance One allele suppresses the expression of the other allele.• Incomplete dominance: phenotype of F1

hybrids is somewhere between the phenotypes of the 2 parental varieties – neither allele completely dominant (White x Red = Pink)

• Codominance, 2 dominant alleles affect the phenotype in separate, distinguishable ways. (Red and white flowers = White and red visible.)

5. MULTIPLE ALLELES Most genes exist in populations in more than two

allelic forms. For example, the four phenotypes of the ABO

blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i.

The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither.

6. PLEIOTROPY

Most genes have multiple phenotypic effects, a property called pleiotropy

For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease

7. Polygenic Inheritance

Polygenic inheritance is an additive effect of two or more genes on a single phenotype

Skin color in humans is an example of polygenic inheritance.

8. DETERMINING THE SEX OF A BABY In humans and other mammals, there are

two varieties of sex chromosomes: a larger X chromosome and a smaller Y chromosome

Only the ends of the Y chromosome have regions that are homologous with the X chromosome

The SRY gene on the Y chromosome codes for the development of testes.

X and Y CHROMOSOMES

Females are XX, and males are XY Each ovum contains an X

chromosome, while a sperm may contain either an X or a Y chromosome.

DIAGRAM TO DETERMINE THE SEX OF A BABY

9. Inheritance of Sex-Linked Genes The sex chromosomes have genes for many

characters unrelated to sex A gene located on either sex chromosome is

called a sex-linked gene In humans, sex-linked refers to a gene on the

larger X chromosome. For a recessive sex-linked trait to be expressed

A female needs two copies of the allele A male needs only one copy of the allele.

Sex-linked recessive disorders are much more common in males than in females.

Some disorders caused by recessive alleles on the X chromosome in humans:

• Color blindness• Duchenne muscular dystrophy• Hemophilia