Genetics & Evolution
Mendelian Genetics
5th Year · 6th Year (Leaving Cert)
- ✓By the end of this lesson students will be able to define key genetic terms such as gene, allele, genotype, phenotype, homozygous, heterozygous, dominant, and recessive.
- ✓By the end of this lesson students will be able to predict the outcomes of monohybrid crosses using Punnett squares and determine genotypic and phenotypic ratios.
- ✓By the end of this lesson students will be able to predict the outcomes of dihybrid crosses (HL) using Punnett squares and probability, applying the Law of Independent Assortment.
- ✓By the end of this lesson students will be able to explain the inheritance patterns of sex-linked traits and solve related genetic problems.
- ✓By the end of this lesson students will be able to apply probability principles to predict the likelihood of specific genotypes and phenotypes in genetic crosses.
Key concepts
A segment of DNA that codes for a specific protein, which in turn determines a particular trait or characteristic.
Alternative forms of a gene, located at the same locus (position) on homologous chromosomes. For example, the gene for height in pea plants can have a 'tall' allele or a 'dwarf' allele.
An allele that expresses its phenotypic effect even when heterozygous with a recessive allele. It masks the presence of the recessive allele. Represented by a capital letter (e.g., 'T' for tall).
An allele whose phenotypic effect is masked by a dominant allele when heterozygous. It is only expressed when an individual is homozygous recessive. Represented by a lowercase letter (e.g., 't' for dwarf).
The genetic makeup of an individual, referring to the specific combination of alleles for a particular gene (e.g., TT, Tt, tt).
The observable physical or biochemical characteristics of an individual, resulting from the interaction of its genotype and the environment (e.g., tall, dwarf, black fur).
Having two identical alleles for a particular gene (e.g., TT - homozygous dominant, tt - homozygous recessive).
Having two different alleles for a particular gene (e.g., Tt). The dominant allele's phenotype will be expressed.
A genetic cross involving a single pair of contrasting traits, used to study the inheritance pattern of one gene.
A genetic cross involving two pairs of contrasting traits, used to study the inheritance patterns of two different genes simultaneously.
A trait whose gene is located on a sex chromosome (typically the X chromosome in humans). These traits show different inheritance patterns in males and females.
A diagram used to predict the genotypes and phenotypes of offspring from a genetic cross by showing all possible combinations of gametes.
Mendel's first law states that during gamete formation, the two alleles for a heritable character separate (segregate) from each other so that each gamete carries only one allele.
Mendel's second law states that each pair of alleles segregates independently of other pairs of alleles during gamete formation. This applies to genes located on different chromosomes or far apart on the same chromosome.
The likelihood or chance of a particular event occurring. In genetics, it is used to predict the frequency of specific genotypes or phenotypes among offspring.
Key facts to remember
- 1Mendel's Laws of Segregation and Independent Assortment form the fundamental principles of classical genetics.
- 2A dominant allele will always express its trait when present, masking the effect of a recessive allele in a heterozygous individual.
- 3Punnett squares are essential tools for predicting the genotypes and phenotypes of offspring from genetic crosses.
- 4For a monohybrid cross between two heterozygotes (e.g., Tt x Tt), the typical phenotypic ratio is 3:1.
- 5For a dihybrid cross (HL) between two double heterozygotes (e.g., BbSs x BbSs), the typical phenotypic ratio is 9:3:3:1.
- 6Sex-linked traits are carried on the sex chromosomes, most commonly the X chromosome, leading to different inheritance patterns in males and females.
- 7Probability is used to quantify the chance of specific genetic outcomes, often expressed as a fraction or percentage.
- 8Genotype refers to the genetic makeup, while phenotype refers to the observable characteristics.
Worked examples
Example 1
In pea plants, the allele for tallness (T) is dominant over the allele for dwarfness (t). A homozygous tall pea plant is crossed with a dwarf pea plant. The F1 generation is then self-pollinated to produce the F2 generation. Determine the genotypic and phenotypic ratios of the F2 generation.
Answer
F2 Genotypic Ratio: 1 TT : 2 Tt : 1 tt F2 Phenotypic Ratio: 3 Tall : 1 Dwarf
Always clearly define your alleles and show all steps, including the Punnett square, for full marks.
Example 2
(Higher Level) In guinea pigs, black fur (B) is dominant over white fur (b), and short hair (S) is dominant over long hair (s). A male guinea pig heterozygous for both traits (BbSs) is crossed with a female guinea pig also heterozygous for both traits (BbSs). Determine the expected phenotypic ratio of their offspring.
Answer
The expected phenotypic ratio of the offspring is 9 Black, Short : 3 Black, Long : 3 White, Short : 1 White, Long.
Remember the Law of Independent Assortment when forming gametes for dihybrid crosses. Each gene's alleles segregate independently.
Example 3
Red-green colour blindness is a sex-linked recessive trait in humans, carried on the X chromosome (X^B for normal vision, X^b for colour blindness). A woman with normal vision, whose father was colour-blind, marries a man with normal vision. What is the probability that their first child will be a colour-blind daughter?
Answer
The probability that their first child will be a colour-blind daughter is 0% (or 0/4).
Pay close attention to the sex chromosomes and how alleles are passed on. Males (XY) express all alleles on their single X chromosome.
Common mistakes
- ✗Confusing genotype (the genetic code) with phenotype (the observable trait).
- ✗Incorrectly determining the gametes produced by parents, especially in dihybrid crosses where all combinations must be considered.
- ✗Failing to use correct notation for sex-linked traits (e.g., writing X^A instead of X^A for the allele on the X chromosome).
- ✗Miscalculating probabilities or ratios, or not expressing them clearly in the final answer.
- ✗Assuming that all traits follow simple Mendelian inheritance, overlooking other patterns like incomplete dominance or codominance (though these are separate topics).
- ✗Not showing all steps in the solution, particularly the Punnett square, which is often required for full marks.
Exam tips
- ★Always clearly define the alleles you are using at the start of your answer (e.g., 'Let T = tall, t = dwarf').
- ★Show all your working, including parental genotypes, the gametes produced, and the full Punnett square. This demonstrates your understanding and allows for partial marks.
- ★Clearly state the genotypic and phenotypic ratios in your final answer, ensuring they are simplified to their lowest terms.
- ★For Higher Level dihybrid crosses, practice forming gametes accurately and constructing the 4x4 Punnett square systematically.
- ★When dealing with sex-linked traits, remember that males (XY) only have one X chromosome, so they express any allele on it, whether dominant or recessive. Females (XX) can be carriers.
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