Cell Division and Genetics

Genetics is the most calculable topic in General Science. The Punnett square questions have a right answer you can work out rather than recall, which makes them worth the few minutes it takes to learn the method.

Mitosis against meiosis

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Two things to hold: mitosis gives two identical cells, meiosis gives four different ones with half the chromosomes.

Humans have 46 chromosomes in 23 pairs in body cells. A sperm or egg cell - together called gametes - has 23, so fertilization restores 46. That is the reason meiosis halves the number - without it every generation would double.

The parts of a chromosome

Before a cell divides, each chromosome copies itself. The two identical copies are sister chromatids, joined at a pinched point called the centromere - which is why a copied chromosome looks like an X. Spindle fibers are the threads that attach at the centromere and pull the chromatids apart. At each end of a chromosome is a telomere, a protective cap, a bit like the plastic tip on a shoelace.

Most of a cell's life is spent in interphase, growing and working, not dividing. The DNA is copied during the S phase of interphase (S for synthesis), which is why each chromosome enters mitosis already doubled.

The phases of mitosis

Worth knowing in order. The mnemonic is PMAT.

Cytokinesis, the division of the cytoplasm, follows and is not one of the four.

Metaphase is middle and anaphase is apart - both mnemonics work on the first letter, which is how the test expects them to be told apart.

What makes meiosis different

Meiosis runs the same four phases twice, and two events are asked about:

• Crossing over happens in prophase I: matching chromosomes from the mother and the father swap sections. It is why siblings are not identical, and it is the main source of the variation meiosis exists to create.
• Meiosis I separates the matching pairs (homologous chromosomes). Meiosis II separates the sister chromatids, just as mitosis does.

When a pair fails to separate properly - nondisjunction - a gamete ends up with an extra or a missing chromosome, and so does any offspring it makes.

Mendel

Gregor Mendel, a monk breeding pea plants in the 1860s, worked out the rules of inheritance before anyone knew what a gene was, and he is the one scientist this topic asks for by name. Two of his conclusions are the whole basis of the Punnett square:

• Each parent passes on one of its two alleles, chosen at random (the law of segregation).
• Different traits are inherited independently of one another (the law of independent assortment).

The vocabulary

TT and Tt look different and produce the same plant. That is the genotype and phenotype distinction, and it is asked directly.

The Punnett square

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The method, for a single trait:

1. Write one parent's alleles across the top, one per column.
2. Write the other parent's down the side, one per row.
3. Fill each box with the letter from its column and the letter from its row.
4. Count.

The cross worth memorizing

Tt crossed with Tt gives TT, Tt, Tt, tt.

• Phenotype ratio: 3 tall to 1 short. Three boxes carry at least one T.
• Genotype ratio: 1 TT to 2 Tt to 1 tt.

3 to 1 is the answer to more Punnett square questions than any other, and recognizing that a question describes two heterozygous parents gets you there immediately.

The other crosses

A recessive trait appears only from tt, so a short plant must have two short parents' alleles. A question saying two tall plants produced a short offspring is telling you both parents were Tt.

Two traits at once (a dihybrid cross, AaBb x AaBb) gives a phenotype ratio of 9 to 3 to 3 to 1. You will not be asked to build that square, only to recognize the ratio.

When neither allele dominates

Not every trait follows the tall-short pattern. Two exceptions are asked:

The words carry the meaning: incomplete dominance means neither wins fully, so they mix; codominance means both dominate together, so both show.

Sex determination and sex-linked traits

Humans have 22 pairs of autosomes and one pair of sex chromosomes.

Females are XX. Males are XY. The egg always carries an X, so the sperm determines the sex of the offspring.

Sex-linked traits - also called X-linked - sit on the X chromosome. Because a male has only one X, a single recessive allele on it is expressed with nothing to mask it. That is why red-green color blindness and hemophilia are far more common in males.

What you can skip

Across all 2,104 General Science questions in our bank:

• Pedigree charts never appear. Punnett squares are the tool the test uses.
• Polygenic traits, linkage maps and centimorgans: zero mentions.
• Named chromosome disorders. Down syndrome and trisomy never appear; what is asked is that nondisjunction gives an abnormal chromosome number.
• The cell cycle's control proteins. Cyclins and cyclin-dependent kinases come up once, at the hardest difficulty. Know that DNA is copied in the S phase and leave the regulation.

Where people lose points

Saying meiosis produces two cells. It produces four.

Saying mitosis halves the chromosome number. Meiosis does; mitosis keeps it.

Swapping genotype and phenotype.

Treating a 3 to 1 ratio as a guarantee rather than a probability.

Forgetting that a recessive phenotype requires two recessive alleles.

Mixing up metaphase and anaphase. Middle, then apart.

Work one in under a minute

In pea plants, yellow seed color (Y) is dominant over green (y). Two heterozygous plants are crossed. What fraction of the offspring are expected to have green seeds?

Heterozygous means Yy, so this is Yy crossed with Yy.

The square gives YY, Yy, Yy, yy.

Green is recessive and needs yy, which is one box in four.

One quarter, or 25 percent.

Note the shortcut: any Tt by Tt cross gives 3 to 1, so the recessive share is always a quarter. Recognizing the cross skips the square entirely.

Where this leads

The chromosomes being divided here are made of the DNA the next lesson covers, and the variation meiosis creates is what natural selection acts on.