Genetic Basis of Inheritance
Easy Overview
Ever wondered why you look like your parents but not exactly like them? That's genetics for you. This chapter is about how traits get passed down from generation to generation — the whole reason you have your mom's eyes but your dad's weird toe.
Mendel's experiments — why peas?
Mendel wasn't just randomly playing with peas. He chose them because they grow fast, have clear traits (tall vs short, green vs yellow), and you can control who mates with who. Smart guy. He crossed pure tall with pure short plants, got all tall in the first generation, then got a mix in the second. That's where the whole dominance idea comes from — tall was just bossing over short.
Dominant vs recessive — the bully and the quiet kid
Think of dominant alleles as the loud kid in class who always gets picked. Recessive ones only show up when the dominant one isn't around. So Tt gives you tall because T (tall) dominates t (short). But tt? That's when short finally gets its moment. Simple, right?
Law of Segregation — alleles break up
During gamete formation, the two alleles for a trait separate. It's like splitting a pair of socks — each gamete gets one sock, not both. That's why kids get one allele from each parent. Mendel figured this out without even knowing what DNA was. Absolute legend.
Law of Independent Assortment — mix and match
Genes for different traits sort independently of each other. Imagine packing two different colored shirts and two pairs of pants. How many outfit combos can you make? That's independent assortment. But heads up — this only works if the genes are on different chromosomes. If they're on the same one, linkage happens.
Linkage and crossing over — when genes stick together
Genes on the same chromosome travel together — that's linkage. They're like students on the same bus. But sometimes during meiosis, chromosomes swap bits (crossing over), and genes get shuffled. The closer two genes are on a chromosome, the tighter they stick. That's how we make gene maps.
Sex determination — XX or XY
In humans, females are XX, males are XY. The mom always gives an X, the dad gives either X or Y. So technically, dad decides the baby's sex. No pressure, dad. There's also sex-linked disorders like color blindness and hemophilia that hang out on the X chromosome — which is why guys get them more often (they only have one X to fall back on).
Key Points
- •Mendel's laws: dominance, segregation, independent assortment
- •Monohybrid cross gives 3:1 ratio, dihybrid gives 9:3:3:1
- •Alleles are different versions of the same gene
- •Linkage breaks independent assortment when genes are close on same chromosome
- •Crossing over during prophase I shuffles genes between homologous chromosomes
- •Sex determination is XX (female) and XY (male)
- •Test cross (cross with recessive parent) reveals unknown genotype
- •Pedigree analysis helps trace genetic disorders in families
Practice Questions
- Explain with a cross how Mendel arrived at the Law of Segregation.
- What's the difference between linkage and crossing over? How does crossing over affect linkage?
- A colour-blind man marries a normal woman. What are the chances their son will be colour-blind?
- Draw a dihybrid cross between two heterozygous parents and explain the 9:3:3:1 ratio.
- How does independent assortment generate genetic variation? When does it not apply?