PSYB20H3 Chap.6 Heredity and Patterns of Inheritance
Heredity and Patterns of Inheritance
From conception to chromosomes
Lecture 3 of PSYB20H3 begins with heredity. Conception occurs when a sperm fertilizes an ovum, forming a zygote with 46 chromosomes, 23 from each parent. Chromosomes are threadlike structures of DNA in the cell nucleus, and a gene is a stretch of DNA carrying a particular set of biochemical instructions.
DNA forms a double helix, replicates itself, and pairs adenine with thymine and cytosine with guanine.
Mitosis, meiosis and twins
Mitosis duplicates a cell's 46 chromosomes and produces two identical daughter cells.
Meiosis produces gametes: the 46 chromosomes are duplicated, maternal and paternal pairs line up, crossing over swaps genes between them, two cells form and then four, each ready to become a gamete with 23 chromosomes. Monozygotic twins develop from one zygote that divides, so they share all inherited characteristics; dizygotic twins develop from two ova fertilized by two different sperm.
Biological sex is set by the 23rd pair, XX in females and XY in males, and the father's sperm determines it.
Genotype to phenotype
The genotype is the genes a person inherits and the phenotype is their observable expression. In simple dominant-recessive inheritance, one allele masks the other. Codominance expresses both alleles equally, as in blood type AB.
Incomplete dominance lets a stronger allele fail to mask all effects of the other, as in sickle cell anemia. Sex-linked traits sit mostly on the X chromosome, so males are more likely than females to show recessive X-linked traits such as colour blindness.
Polygenic traits such as height and temperament depend on many genes, and epigenetic changes such as methylation alter expression without changing DNA.
Solving a cross
The lecture's dimples examples show the method. Write each parent's possible gametes, fill the four cells of a Punnett square and count the cells that carry at least one dominant allele.
A DD parent and a dd parent can only produce Dd children, so all show dimples. A Dd parent and a dd parent produce half Dd and half dd.
Two Dd parents produce one DD, two Dd and one dd, so three in four children are expected to show the dominant trait.
Where marks are lost
The common errors are confusing genotype ratios with phenotype ratios, forgetting that a heterozygous child shows the dominant trait, and treating a probability as a guaranteed count within one family.
Ordering questions on meiosis and items asking what a 23-chromosome sperm means are also frequent; a sperm with 23 chromosomes is entirely typical.
What this chapter covers
- 01
Chromosomes, genes and DNA
- 02
Mitosis and meiosis
- 03
Crossing over
- 04
Monozygotic and dizygotic twins
- 05
Genotype and phenotype
- 06
Dominant, recessive and codominant alleles
- 07
Sex-linked and polygenic inheritance
- 08
Epigenetics
Worked example · free
Expected dimples from two Dd parents
- 1Each parent contributes D or d, giving four equally likely cells: DD, Dd, Dd, dd.
- 1Three cells contain D, so the chance of dimples is 3 in 4, or 75%.
- 1Genotypes occur in the ratio 1 DD : 2 Dd : 1 dd.
Key terms
- Meiosis
- Meiosis is the cell division that produces gametes, each carrying 23 chromosomes after crossing over has mixed parental genes.
- Monozygotic Twins
- Monozygotic twins are identical twins formed when one zygote divides into two genetically identical individuals.
- Codominance
- Codominance is inheritance in which two alleles are expressed equally strongly, as in blood type AB.
- Epigenetics
- Epigenetics is the modification of gene expression without any change to the DNA itself.
- Polygenic Trait
- A polygenic trait is a characteristic shaped by many pairs of alleles, such as height or temperament.
Heredity and Patterns of Inheritance FAQ
How many chromosomes does a typical sperm or egg carry?
A typical sperm or ovum carries 23 chromosomes, half the usual number. At conception the two combine to give the zygote 46 chromosomes, 23 from each parent. A fertility result reporting 23 chromosomes in sperm therefore describes a typical male, a point the lecture tests directly.
Why are males more likely to show X-linked recessive traits?
Most sex-linked genes sit on the X chromosome. A female has two X chromosomes, so a typical allele on one can mask a recessive allele on the other. A male has only one X, so any recessive allele it carries is expressed, as with colour blindness.
What is the difference between codominance and incomplete dominance?
In codominance, both alleles are expressed equally strongly, as in blood type AB. In incomplete dominance, one allele is stronger but fails to mask all the effects of the other, so a heterozygous person shows a partial effect. The lecture uses sickle cell anemia as its example of incomplete dominance.
Exam move
Practise five crosses from scratch, varying the parents' genotypes, and state both the genotype and the phenotype ratio for each. Then order the meiosis steps from memory and explain in one sentence why two siblings from the same parents are not genetically identical.
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