Genetics Principles and Concepts Quiz

Genetics and Variation

• Genetics is the study of heredity, the process where parents pass genes to their children.
• Children inherit genes from their biological parents, which express traits like physical characteristics, talents, and genetic disorders.
• Most human cells contain 46 chromosomes.
• 2 sex chromosomes (X, Y); XY in males; XX in females.
• 22 pairs of chromosomes are called autosomes.
• A gene is a segment of DNA located on a specific site on a specific chromosome that codes for a specific protein.
• Chromatin is a thread-like structure of DNA and proteins that carries genes.
• A chromosome is a condensed form of chromatin.
• DNA is deoxyribonucleic acid, containing the genetic instructions for biological development.
• DNA packaged tightly into structures called chromosomes.

Genetic Concepts

• Locuses are locations of genes on the chromosome.

• Alleles are variant forms of a gene.

• Individuals with two copies of the same allele are homozygous for that trait.

• Individuals with different alleles are heterozygous for that trait.

• Dominant alleles are expressed even if paired with a recessive allele.

• Recessive alleles are only expressed when paired with another recessive allele. Dominant alleles are usually capital letters while recessive alleles are lowercase letters.

• The allele for brown eyes is dominant, while the allele for blue eyes is recessive.

Genotypes vs Phenotypes

• Genotype is the set of alleles an individual has for a specific trait.
• Phenotype is the observable physical trait of an individual, like hair colour, weight etc., or the presence/absence of a disease.

Gregor Mendel

• Born in 1822 in Czechoslovakia.
• Became a monk in 1843.
• Taught biology and had an interest in statistics.
• Studied at the University of Vienna.
• After returning to the monastery, continued to teach and worked in the garden.
• Between 1856 and 1863, he grew and tested over 28,000 pea plants.

Mendel's Peas

• Peas are easy to grow.
• Peas have easily identifiable traits.
• Mendel could test large numbers of samples.

Mendel's Experiments

• Studied one characteristic at a time.
• Used 2 contrasting pure breed parent plants (P generation).
• Cross-pollinated the two plants and collected the seeds.
• Planted the seeds.
• The hybrid offspring are called the F₁ generation.
• Self-pollinated the F₁ generation, the offspring of this group are called the F₂ generation.

Mendel's Conclusions

• Mendel examined seven traits and always got the same results.
• The F₁ always showed only one of the two parent traits and always the same.
• The trait that "disappeared" in the F₁ generation reappeared in about 25% of the F₂'s.
• Traits did not blend. They remained unchanged from one generation to another.
• Traits are not always expressed.
• Alleles always expressed are called dominant alleles.
• Alleles not expressed if a dominant allele is present are recessive alleles.

Mendel's Laws

• The Law of Dominance
• The Law of Segregation
• The Law of Independent Assortment

Law of Dominance

• In a cross of parents who are pure for contrasting traits, only one form of the trait appears in the next generation.
• Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.

Law of Segregation

• Two members of a gene pair segregate into the gametes.
• Half the gametes carry one member of the pair, and the other half carry the other member of the pair.
• This law is true in most cases.

Punnett Square

• A Punnett square is a diagram used to predict the genetic makeup of offspring from individuals with particular genotypes.
• Gametes are placed above and to the left of the square.
• Offspring are placed in the square.
• Capital letters (Y) represent dominant alleles.
• Lower case letters (y) represent recessive alleles.

Examples of Traits

• Some traits are inherited, some are environmental, and some are both.

Discontinuous Variation

• You either have the condition or you do not.
• There are no intermediate states.
• A single gene gives the characteristic for discontinuous variation.
• Discontinuous variations are entirely genetically controlled.
• They cannot be altered by external conditions.

Continuous Variation

• Continuous variation describes situations where there are many intermediates between the extremes.
• Either many genes are involved in that characteristic, or the genes are operating with environmental influences.

Some Exceptions to Mendel's Principles

• Some alleles are neither dominant nor recessive.
• Many traits are controlled by more than one gene (polygenic traits).

Incomplete Dominance

• F1 hybrids have a phenotype somewhere between the phenotypes of the two parents.
• Incomplete dominance occurs in the snapdragon flower colour.

Multiple Allele Inheritance

• Three or more alleles of a given gene exist.
• The human ABO blood group is controlled by three alleles: IA, IB, and i.
• IA and IB are codominant.
• Both IA and IB are dominant over i.

Co-dominance: ABO Blood Type

• IA codes for an enzyme that attaches galactose (G) to RBCs.
• IB codes for an enzyme that attaches acetyl galactosamine (A) to RBCs.
• i codes for an inactive enzyme.

Sex-linked Inheritance

• Sex-linked genes are located on X chromosomes.
• The Y chromosome does not carry these genes because it is shorter.
• Haemophilia and Colour blindness are examples of X-linked traits.
• In a colour blind man, the recessive colour blindness gene on the X chromosome will be expressed.
• A normal female carrying a recessive colour blind gene is a carrier.
• If a normal female carries a recessive colour blindness gene on her X chromosome, she is a carrier, but not colour blind herself.

Examples of Questions / Problems

• Predict the genotype and phenotype of offspring.
• Show how colour blindness is inherited.
• Show the possible blood groups of offspring for specific parent blood groups.
• What are the probabilities of a child having hemophilia?
• Probability of a daughter being a carrier of hemophilia.
• Probability that a daughter would have hemophilia.
• Probability that a son will have hemophilia when specific parents are known.