Heredity · Lesson 5 of 7
How do these Traits get Expressed?
“Genes do not wave magic wands; proteins and cell processes do the actual work.”
• Explain how genes influence traits through proteins. • Trace the pathway from a DNA difference to a visible characteristic. • Relate enzyme efficiency and hormone production to plant height. • Explain why body cells carry chromosome pairs but germ cells carry one set. • Describe how fertilisation restores the normal chromosome number.
A gene is not a tiny visible version of a trait. A plant does not contain a miniature instruction labelled “tall.” Instead, DNA provides information for proteins. Those proteins build structures or control chemical processes, and the results of those processes contribute to the characteristic we observe.
DNA As The Information Source For Proteins
Cellular DNA contains information used to make proteins. A gene is a section of DNA that provides information for a functional product, often a protein. Proteins can form cellular structures, transport substances, act as receptors or function as enzymes that control reactions. Traits emerge from the organised effects of these molecules.
Plant Hormones And Plant Height
Plant growth is influenced by hormones. Producing a hormone requires a sequence of chemical reactions, each controlled by enzymes. If a gene carries information for an efficient enzyme in this pathway, enough hormone may be produced to support greater stem growth. If an alternative gene form produces a less efficient enzyme, less hormone may be formed and the plant may remain shorter.
This example explains why a change in DNA can influence an observable trait. The DNA difference changes a protein; the altered protein changes a biochemical process; the changed process alters growth. Environment can still influence the result because water, minerals and light also affect growth. Genes guide biological processes rather than fixing every outcome in isolation.
Solving Gene-To-Trait Reasoning Problems
Begin with the changed level named in the problem: gene, protein, process or trait. Move through the pathway one link at a time and use cautious language such as “may reduce” when the evidence is incomplete. Do not jump directly from DNA to appearance, and do not assume that every altered gene is harmful.
Problem
Explain how an efficient enzyme can contribute to a tall plant.
- 1.The gene provides information for the enzyme.
- 2.The enzyme works efficiently in a hormone-producing pathway.
- 3.More growth-promoting hormone is produced.
- 4.The hormone stimulates greater stem growth.
- 5.The observable phenotype is tallness.
Problem
Trace the effect of a gene alteration that reduces enzyme efficiency.
- 1.The DNA sequence of the gene is altered.
- 2.The protein produced may have a changed shape or activity.
- 3.The enzyme-controlled reaction proceeds less efficiently.
- 4.Less growth hormone may be produced.
- 5.Reduced growth can contribute to a short phenotype.
Problem
Two plants have the same height-related genotype but grow to different heights. Explain.
- 1.The shared genotype can produce similar growth-related proteins.
- 2.One plant may receive less water, light or mineral nutrition.
- 3.Its cellular processes and growth are limited by the environment.
- 4.The final phenotype therefore reflects genetic information interacting with conditions.
- 5.Different height does not automatically mean a different allele combination.
Chromosomes And Equal Parental Contribution
For Mendel’s explanation to work, both parents must contribute a copy of each kind of gene. Genes are located on chromosomes, which are separate pieces of DNA rather than one unbroken thread. Most body cells carry two copies of each chromosome: one of maternal origin and one of paternal origin. The two copies carry corresponding genes, although their alleles may differ.
Formation Of Germ Cells
A germ cell must receive only one chromosome from each pair. The chromosome entering a particular germ cell may be the maternal or paternal member of that pair. Because different chromosome pairs separate independently, germ cells can carry new combinations of parental chromosomes. This chromosome behaviour provides a physical explanation for the independent inheritance observed in Mendel’s crosses.
Restoration During Fertilisation
If germ cells carried the normal two sets, fertilisation would double the chromosome number in every generation. Instead, germ-cell formation reduces the number to one set. When two gametes combine, the zygote receives one set from each parent and the normal two-set condition is restored. This maintains chromosome-number stability across generations while allowing genetic combinations to change.
Problem
A species has 12 chromosomes in each body cell. How many are in a gamete and a zygote?
- 1.Given: body-cell number 2n = 12.
- 2.A gamete carries half, so n = 6.
- 3.Fertilisation combines 6 + 6.
- 4.The zygote has 12 chromosomes.
- 5.The original species number is restored.
Problem
A gamete contains 9 chromosomes. Find the body-cell and zygote numbers.
- 1.Given: gamete number n = 9.
- 2.Body cells contain two sets: 2n = 18.
- 3.A second gamete also contributes 9 at fertilisation.
- 4.Zygote number = 9 + 9 = 18.
- 5.The zygote matches the body-cell chromosome number.
Problem
Explain why a zygote would have 40 chromosomes if two unreduced gametes from a species with 20 body-cell chromosomes fused.
- 1.Normal body cells have 2n = 20, so normal gametes should have n = 10.
- 2.An unreduced gamete incorrectly retains 20 chromosomes.
- 3.Two such gametes contribute 20 + 20.
- 4.The resulting zygote has 40 chromosomes.
- 5.This demonstrates why reduction during germ-cell formation is required for stability.
A gene does not directly become a visible trait, and fertilisation does not halve chromosome number. Germ-cell formation halves the number; fertilisation restores it.
Quiz
What is a gene?
How can a gene influence plant height?
Why do body cells usually carry chromosome pairs?
What happens to chromosome number during normal germ-cell formation?
What does fertilisation restore?
Practice Problems
- Problem: Trace tallness from gene to phenotype. Solution: Gene information produces an enzyme; the enzyme supports hormone synthesis; the hormone promotes stem growth; greater growth produces a tall phenotype.
- Problem: A body cell has 24 chromosomes. Find the gamete and zygote numbers. Solution: Gamete = 12; zygote = 12 + 12 = 24.
- Problem: A gamete has 7 chromosomes. Find 2n. Solution: 2n = 2 × 7 = 14 chromosomes in body cells and the zygote.
- Problem: Explain independent chromosome combinations in gametes. Solution: Each germ cell receives one chromosome from every pair, and that chromosome may be maternal or paternal; different pairs separate independently.
- Problem: Explain why genes do not act alone in determining plant height. Solution: Genes influence proteins and growth pathways, while water, light and nutrients also affect how much growth occurs.
Key Takeaways
• Genes are sections of DNA that provide information for functional products. • Proteins and enzymes connect genetic information to cellular processes and traits. • Plant height can depend on enzyme-controlled hormone production. • Genotype influences phenotype through biological processes, not by directly creating appearance. • Body cells carry corresponding chromosome copies from both parents. • Germ cells receive one chromosome from each pair. • Fertilisation combines two one-set gametes and restores the normal chromosome number. • Chromosome separation explains equal parental contribution and independent inheritance.