Biology, Chapter 6 Study Guide
What’s Chapter 6 about? Mendel and Meiosis or how genes are mixed so that organisms, such as us,
have genetic variation—so we don’t all look the same. Why? So that there is diversity. Why is this
good? So that organisms receive traits that will help them survive.
Pre-fixes, roots, and suffixes are the keys to learning Biology. Chromo or Chrom means color. Some
means body. Look at the colorful chromosomes (color bodies) on page 169.
Key Concept: gametes have half the number of chromosomes that body cells have. ? Unlike in mitosis
(mi- means move, like migrate), where the DNA moves to the ends of the cell to then divide and make
exact duplicates, meiosis is process of genetic mixing, which requires first making gametes: sex cells.
Somatic cells (body cells), make up all of you—your tissue, organs, eyeballs, and ear lobes. Somatic cells
have 23 pairs of chromosomes, which is 46 total.
Somatic cells are diploid. Which means two versions. (di- is two). Body cells therefore all have two
versions.
What about pairs of chromosomes? They’re called homologous chromosomes (homo- means same, log
means category or list; a list of features with different versions. Like at a grocery store there is milk, but
different versions: whole milk, 2% milk, chocolate milk, goats’ milk) because, even though they are not
identical, they carry the same genes, but different versions of the genes. One comes from mommy the
other daddy.
Genes (Gen- means create, so genes are the basis to create) are on stretches of DNA, which are on
chromosomes. Think of a song being in a certain place on a CD. On another copy of the CD the song
would be in the same place. And you may have more than one CD, with more songs.
Autosomes are body cells. Chromosomes from #1 to #22 contain the genes for our body and therefore
are called autosomes (our – body). Why are they called diploid?
Gametes (give traits); in humans, the egg and sperm have half the number—haploid--of chromosomes:
23. (Ha-means half).
This is so that during fertilization, the genes re-combine (they are fuzed—permanently put together) to
make a full set again (46)!
The 23rd pair of chromosomes in humans determines your sex, your maleness or femaleness. The pair is
either XX – female or XY – male. During fertilization you either get XY or XX, one X or Y from your mom
and one X or Y from your Dad.
Meiosis is a two stage process to separate, first, the chromosomes, and second, the chromatids, so that
gametes (egg and sperm) can be made. The first stage (Meiosis I) separates the chromosomes. The
second stage (Meiosis II) separates the chromosomes into chromatids. Look on page 175 and you will
see that the process is like mitosis, but times 2. The final result is haploid gametes (egg and sperm with
half the genes ready for fertilization).
This is why it is also called gametogenesis (generate gametes). In making eggs there are leftovers called
polar bodies.
Genetics is the study of inheritance.
Mendel figured out that one trait is inherited from the mother and one from the father. How did he do
this?
He made purebred (100% one breed, like Chihuahuas or Poodles) flowers by isolating them: pure purple
and pure white.
He then crossed (cross means to “X,” or to put one across another, or to mate) the white and purple and
got 100% purple. How is this possible? He believed one trait was dominant (it overrode the other trait).
Even though the other gene, genotype, is there we don’t see it. We see the only phenotype (physical
type). He called this the first or F1 generation.
Next, he crossed (mated) these. Even though they were all purple, he thought he might get something
different in the next generation. And he did. He got a 3:1 ratio (proportion) of purple to white. This
was the F2 generation.
You have tested this with a Punnett’s square. PP x pp = 100% purple (phenotype—what you physically
see). Try it. Then Pp x Pp = 3 purple to every 1 white.
From this Mendel came up with the law of segregation:
1. Organisms inherit one gene from each parent
2. Organisms donate one gene
The same gene can have many versions. These alternatives are called alleles.
If a pair of chromosomes have the same alleles they are called homozygous (same piece). For example,
all dominant, or all recessive. PP or pp.
If a pair of chromosomes have different alleles they are heterozygous (different piece). One dominant
and one recessive. Pp.
To test one allele do a monohybrid cross, such as for plant height, tall or short.
Also, using a testcross, we can “backwards” figure out what the genotypes are. Using this, like we did in
class, we can predict an unknown genotype by crossing it with a known genotype.
A dihybrid cross is crossing two traits at the same time. This requires a bigger punnett’s square and
concentration but shows how much gene variety there can be with just two traits.
Mendel’s second law was the law of independent assortment, which means that genes work by
themselves. Just because someone has blonde hair doesn’t mean they will have blue eyes. Each gene is
independent. Just like flipping one quarter has no effect on flipping another quarterly
The chance of getting some trait is determined by probability. If you flip a quarter what is the chance it
will be “heads?” One out of two. There are two possibilities but you will only get one. Mathematically
we say 1/2, one out of two, or 50%.
Another way genes are mixed is when homologous chromosomes “tango.” We call this crossing over.
Literally, chromosomal material is exchanged from one chromosome to the other. And if a big piece is
exchanged for another big piece, it is likely that the genes near each other will cross over together.
Therefore, genes close together tend to be inherited together and this is called genetic linkage.