Genetics and Inheritance Notes

Knowledge Strands

• Life at molecular, cellular, and tissue level.
• Diversity, change, and continuity.

Topic 5: Genetics and Inheritance

• Unit 1: Genes and genetic concepts
  • 1.1 Genes
  • 1.2 Dominant and recessive alleles of a gene
  • 1.3 Gregor Mendel - father of genetics
• Unit 2: Inheritance and variation
  • 2.1 Monohybrid crosses
  • 2.2 Dihybrid crosses
• Unit 3: Sex chromosomes
  • 3.1 Sex-linked alleles and sex-linked diseases
• Unit 4: Mutations
  • 4.1 Harmless and harmful gene mutations
  • 4.2 Chromosomal aberrations
  • 4.3 Useful mutations and natural selection
  • 4.4 Genetic lineages
• Unit 5: Genetic engineering
  • 5.1 Biotechnology and genetically modified (GM) organisms
  • 5.2 Genetically modified animals, cloning, and stem cell research
• Unit 6: Mitochondrial DNA and the tracing of genetic links
• Unit 7: Paternity testing and DNA fingerprinting (forensics)
  • 7.1 DNA fingerprinting (forensics)
  • 7.2 Paternity testing

Introduction to Genetics

• Genetics is the science of heredity, explaining how characteristics are passed from one generation to the next.
• Topic explores inheritance of characteristics, including genetic diseases.
• Geneticists work to:
  • Replace damaged/missing genes.
  • Produce needed products for those with diseases.
  • Improve crops for better production.

Activity 1: Prior Knowledge Check

• 1.1 Gene: Basic unit of heredity.
• 1.2 Chromosome: Structure containing DNA.
• 1.3 Homologous chromosome: Pair of chromosomes with identical genes.
• 2.1 Genes passed from parents to children through meiosis and sexual reproduction.
• 2.2 Genes determine characteristics by coding for proteins.
• 3. Example of inherited disease: Cystic Fibrosis.
  • Affects health by causing lung infections and digestive issues.
• 4. Genetically engineered foods:
  • Bt corn (insect-resistant).
  • Golden rice (vitamin A enhanced).

Careers Focus: Genetic Counselor

• Job description:
  • Provides information about genetic conditions and inheritance.
  • Draws up family trees (pedigrees).
  • Discusses causes, inheritance, and risks.
  • Suggests genetic testing and offers emotional support.
• Education:
  • Bachelor of Science Honours (BSc Hons) in Genetics or related field.
  • Master of Science (MSc) or Master of Education (MEd) in Genetic Counselling.
• Skills:
  • Understanding of genetics.
  • Awareness of moral/ethical issues.
  • Communication skills, compassion, objectivity.

Unit 1: Genes and Genetic Concepts

• Key Questions:
  • What are genes and alleles? What information do they carry?
  • What are dominant and recessive alleles?
  • Why was Mendel's work important?
• Key words:
  • Locus: position of a gene on a chromosome.
• Gene:
  • Section of DNA in a chromosome that carries a code about a particular characteristic in an organism.
  • Code determined by unique arrangement of nucleotides.
  • Order of nucleotides transcribed to mRNA.
  • Determines amino acids joined together, therefore proteins produced.
  • Proteins form structures, enzymes, or hormones.
• Genes control every characteristic and essential function in the body.
• Each gene is found in a particular position, or locus, on a chromosome.
• Somatic/body cells are diploid: have pairs of chromosomes (homologous pairs).
• On each homologous pair (except sex chromosomes), there are identical genes.
• Gene in a particular locus on the chromosome from the mother is matched by the same gene at the same locus on the chromosome from the father.
• These two genes code for the same characteristic, for example, hair color.

1.2 Dominant and Recessive Alleles of a Gene

• Gene mutation: When a gene mutates by changing a small part of its DNA code, then the instruction it carries also changes.
• Alleles: Different variations of the same gene, carry different information about the same characteristic.
• Multiple alleles: More than two alleles for the same gene in a population (e.g., blood group).
• Individuals have only two alleles for each gene, one from each parent.
• Dominant allele: Masks the expression of the recessive allele when both are present.
• Recessive allele: Only expressed when no dominant allele is present.
• Dominant allele determines phenotype in both homozygous and heterozygous individuals.
• Recessive allele determines phenotype only in homozygous individuals.
• Dominant allele is not always the most common allele.

1.3 Gregor Mendel – Father of Genetics

• Gregor Mendel (1822-1884):
  • Austrian monk who discovered basic principles of heredity.
  • Observed variation in plants and wondered why.
  • Experimented with pea plants over eight years.
  • Used purebred plants with contrasting traits (e.g., tall/dwarf, yellow/green seeds).
  • Recorded results from over 10,000 plants.
  • Observed offspring of purebred cross looked the same.
  • Self-pollination led to specific ratios in offspring.
  • Suggested 'factors' (genes) control characteristics.
• Purebred plant: Plant that, when it pollinates itself, produces offspring that always look like the parent plant.
• Scientist did not understand his findings until some time after his death when chromosomes were discovered.

Unit 2: Inheritance and Variation

• Key questions:
  • How are characteristics inherited in a monohybrid cross and in a dihybrid cross?
  • What is complete dominance, incomplete or partial dominance, and codominance?
  • How can we draw genetic diagrams to show the inheritance of characteristics?

2.1 Monohybrid Crosses

• Monohybrid cross: Mating between monohybrid offspring of homozygous parents differing in alleles for one gene.
• Monohybrid offspring: Have one dominant and one recessive allele for that gene.
• Genotype: Genetic make-up or alleles present.
• Phenotype: Visible expression of the genotype.
• Homozygous: Alleles of a gene are the same.
• Heterozygous: Two alleles of a gene are different.
• Purebred: Homozygous for a characteristic.
• Hybrid: Heterozygous for a characteristic.

Meiosis and Segregation of Alleles

• Offspring receive one allele from each parent for each characteristic.
• Meiosis: Diploid parent cells produce haploid gametes.
• Gametes: Only one allele of a gene.
  • Homologous chromosomes separate.
  • Each gamete receives only one allele of a pair

Genetic Diagrams

• Genetic diagram: Shows genotypes and phenotypes of a cross between two parents.
  • Generations:
    • P (parental): Parent generation.
    • $F_1$ (1st filial): First generation of offspring.
    • $F_2$ (2nd filial): Second generation of offspring.
  • Alleles:
    • Represented by capital (dominant) and small (recessive) letters (e.g., T for tallness).
    • Two alleles for each characteristic.

Punnett Square

• Diagram used to predict outcome of crossing different alleles.
• List alleles of female gametes along the top.
• List alleles of male gametes down the side.
• Combine alleles in each box.

How to Draw Genetic Diagrams

• Write 'gametes' and $F_1$.
• Decide letters for alleles.
• Fill in given phenotype/genotype information.
• Draw Punnett square.
• Fill in missing details.
• Chance factor: genetic crosses, chance determines which male gamete fertilizes the female gamete.
• There is a 3:1 chance that the offspring will be tall or short.
• 75% chance of the offspring being tall;
• 25% chance of the offspring being short.

Complete Dominance

• Alleles for the traits were either dominant or recessive alleles.
  • Alleles for tall pea plants (T) were dominant over alleles for dwarf pea plants (t).
  • Alleles for yellow seeds (Y) were dominant over alleles for green seeds (g).
• The dominant allele blocks the action of the recessive allele so that only the trait
  that the dominant allele codes for is formed.
• So, if you have a heterozygous pea plant such as Tt, only the allele for tallness will be active. As a result, the pea plant will be tall.

Incomplete (or Partial) Dominance

• Neither is dominant, offspring show a blend.

Codominance

• Both alleles expressed in phenotype.

Example of Codominance - Blood Groups

• Four blood groups: A, B, AB, and O.
• Blood group determined by three alleles: $I^A$, $I^B$, and i.
• $I^A$ and $I^B$ produce antigens (A and B).
  • Genotypes:
    • Blood group A: $I^AI^A$ or $I^Ai$
    • Blood group B: $I^BI^B$ or $I^Bi$
    • Blood group AB: $I^AI^B$
    • Blood group O: ii

2.2 Dihybrid Crosses

• Dihybrid cross definition: A mating or genetic cross between the dihybrid offspring of two homozygous parents that differ in their alleles for two genes, one parent having two dominant alleles for both genes and the other having two recessive alleles for both genes.
• Dihybrid definition: An organism that is heterozygous for two genes that we are interested in each gene
  has two different alleles. The dihybrid is the offspring of purebred or homozygous parents that differ in two characteristics of interest.
• Linked alleles: Alleles of the two genes are on the same chromosome.
• If the alleles of the two genes are on different chromosomes, they rearrange
  themselves independently during gamete formation.
• Independent assortment of alleles: the alleles are on separate chromosomes and so are independent of one
  another and can rearrange themselves in different ways during meiosis to form four possible combinations of alleles in the gametes
• Dihybrid Cross Phenotype Ratio: 9:3:3:1

Unit 3: Sex Chromosomes

• Key Questions:
  • How do the sex chromosomes determine whether a child is male or female?
  • Why do certain sex-linked traits appear more often in males than females?
  • How are sexually-linked genetic diseases transmitted from parents to offspring?
• Key Words:
  • Sex chromosomes: The pair of chromosomes that determine your sex.
  • X chromosome: The sex chromosome in males (one X) and females
    (two X)
  • Y chromosome:
    The very short sex chromosome in
    males only
• Each human has 23 pairs of chromosomes in each body cell.
• One pair, the sex chromosomes, determines whether you are male or female.
• There are two sex chromosomes, a long chromosome called the X chromosome anda short chromosome called the Y chromosome in human cells.
• In females, there are two X chromosomes.
• In males there is an X chromosome and a Y chromosome.
• Sex determination genetic diagram:
  • Female (XX) and Male (XY); produces either a male or female (50% probability).

3.1 Sex-Linked Alleles and Sex-Linked Diseases

• Occur more often in males than in females due to structure of X and Y chromosomes.
• Few genes on Y chromosome, many on X chromosome.
• Recessive allele on X chromosome:
  • Males: Effect is seen as they have only one X chromosome.
  • Females: Effect seen only if both X chromosomes have the allele.
    • Carrier: Female with one recessive allele; does not have the disease but can pass it on.
• There are more than 120 sex-linked traits in humans that are caused by sex-linked alleles on the X chromosome.
• Recessive alleles can be harmless or harmful/lethal.

Color Blindness

• More common in men than women, recessive X-linked.

Hemophilia

• Sex-linked disease preventing blood clotting.

Unit 4: Mutations

• Key Questions:
  • What causes chromosomal aberrations and gene mutations?
  • Which mutations are harmless and which are harmful?
  • Which genetic diseases are caused by lethal alleles?
  • How do these alleles affect the body?
• Key Words:
  • Gene mutation: a change in the DNA structure of a gene, resulting in different information being provided about a characteristic
  • Harmless mutations: mutations that do not affect the functioning of the body
• Changes to DNA:
  • Gene Mutation: change in DNA structure
  • Caused by radiation, drugs, etc.
  • Nucleotide loss or change

Chromosomal Aberrations

• Also called chromosomal mutations.
• Changes in chromosome structure or part of chromosome containing several genes.
• Can be caused by errors in mitosis or meiosis or by damaging agents such as radiation.
• Leads to gene loss, ineffectiveness or relocation.
• Cause certain genetic diseases (e.g., cri du chat syndrome).

4.3 Useful Mutations and Natural Selection

• Mutations can be beneficial to organisms.
• Natural selection and evolution is based on the principle that during our evolutionary history, the genes in organisms mutated, forming new alleles for that gene.
• If the mutation was beneficial, that
  organism would have a selective advantage.
• People who have the mutant allele therefore stay warmer than the
  people who do not have this allele. This is an advantage in very cold climates, as
  people who have this allele are more likely to survive the cold weather.
• Natural selection : the allele in these populations

4.4 Genetic Lineages

• Genetic Lineages definition: Lines of inheritance, or the way the alleles of genes
  are passed from generation to generation in a family.
• Family tree: A diagram that shows the links
  between several generations; also called
  a pedigree diagram
• Drawing a family tree:
  |= a male
  O a female
  A clear square or circle shows a normal individual and A shadded is affected