Patterns of Monohybrid Inheritance

Homozygous

Having two identical alleles for a gene (e.g. AA or aa).

Heterozygous

Having two different alleles for a gene (e.g. Aa)

Dominant,

An allele that is expressed in the phenotype when present, even if only one copy is inherited (A masks a).

recessive

An allele that is only expressed in the phenotype when two copies are present (aa); masked by a dominant allele.

genotype

the genetic makeup of an organism for a particular trait (the alleles it carries, e.g. Aa).

phenotype

The observable characteristics or traits of an organism (e.g. brown eyes), resulting from the interaction of genotype and environment

12. Describe the role of genes in the process of inheritance.

Genes are segments of DNA that code for proteins or functional RNA. Each gene occupies a specific position (locus) on a chromosome. During reproduction, parents pass on copies of their genes to their offspring via gametes. The combination of alleles inherited from both parents determines the offspring's genotype, which influences their phenotype. Different alleles can produce different versions of a trait (e.g. different eye colours), so genes are the fundamental basis of inherited characteristics

13. Describe the role of gametes in sexual reproduction.

Gametes (sperm and eggs) are haploid sex cells produced by meiosis. Each gamete carries one set of chromosomes (one copy of each gene). During fertilisation, a sperm and an egg fuse to form a diploid zygote, restoring the full chromosome number. Gametes therefore: Ensure offspring receive genetic material from both parents. Allow mixing of alleles, increasing genetic variation in the population.

14. Describe how genetic information from both parents is passed on to offspring by fertilisation, including explaining how biological sex is determined.

Each parent produces haploid gametes by meiosis, containing one copy of each chromosome. At fertilisation, one sperm (from the father) fuses with one egg (from the mother), combining their chromosomes to form a diploid zygote. The zygote therefore has: One set of chromosomes (and alleles) from the mother. One set from the father. This combination determines the offspring's genotype. Sex determination (in humans): Eggs always carry an X sex chromosome. Sperm carry either X or Y. If an X sperm fertilises the egg → XX (genetically female). If a Y sperm fertilises the egg → XY (genetically male).

15. Explain how heredity and the environment can affect phenotypes.

Phenotype is influenced by both genetic factors (heredity) and environmental factors: Heredity: Different alleles can produce different proteins, leading to different traits (e.g. genes for pigment affect skin or flower colour). Environment: Conditions such as nutrition, temperature, light, chemicals, and lifestyle can modify how genes are expressed. For example, identical twins (same genotype) can differ in height or weight due to diet and exercise. Many traits are polygenic (controlled by multiple genes) and multifactorial (genes + environment), so phenotype is often the result of complex interactions between inherited DNA and environmental influences.

16. Use Punnett squares to predict the proportion of genotypes and phenotypes of given crosses.

A Punnett square is a grid used to show all possible allele combinations from a genetic cross: Write one parent's gametes (alleles) across the top (e.g. A and a). Write the other parent's gametes down the side. Fill in each box by combining the alleles from the top and side. From the completed grid, count the number of each genotype (e.g. AA, Aa, aa) and then determine the phenotypes (e.g. dominant vs recessive trait). This allows you to predict the expected ratios or proportions of offspring genotypes and phenotypes

18. Identify the following pattern of inheritance: autosomal dominant

Gene is on an autosome (non-sex chromosome). Only one copy of the dominant allele is needed to show the trait (A_). Trait often appears in every generation; affected individuals usually have at least one affected parent.

18. Identify the following pattern of inheritance: autosomal recessive

Gene is on an autosome. Trait is only expressed when an individual is homozygous recessive (aa). Can skip generations; unaffected parents (carriers, Aa) can have affected children (aa).

18. Identify the following patterns of inheritance: sex-linked recessive.

(usually X-linked recessive): Gene is on the X chromosome. Recessive allele is expressed in males with just one copy (XᵃY) because they have only one X. Females must be homozygous recessive (XᵃXᵃ) to show the trait; heterozygous females (XᴬXᵃ) are carriers. More common in males (e.g. red-green colour blindness, haemophilia).