Download Chapter 11 and 12 Genetics is the scientific study of heredity

Chapter 11 and 12
Genetics is the scientific study of heredity. Heredity is the passing of traits (characteristics) from generation to
generation.
Gregor Mendel is considered to be the father of genetics. He studied inheritance using pea plants. Pea plants
are usually self pollinating (meaning they reproduce within the same plant, rather than using two different pea
plants). But Mendel wanted to get seeds that inherited traits from two different parents (plants), so he acted like a
bee- hand pollinating specific plants so he would know which two plants were the parents and could see how
traits were inherited.
Mendel would cross two plants with different traits, such as height (one tall plant, one short plant). Then, he
would grow the resulting seeds and see what kind of plant he got. The offspring were hybrids- crosses between
parents with different traits. The first generation is the F1 generation. Cross plants from the F1 generation, and
those offspring would be F2 generation, and so on.
In each group, the hybrid plants only looked like one of the parents. For example, all of the hybrids were tall in
the F1- none were short. From this observation, Mendel made 2 conclusions:
1. Traits are determined by genes. One gene with two different blueprints controlled each trait. Each form of the
gene is called an allele.
2. Principle of Dominance- some alleles are dominant (show up in the hybrid) and some are recessive (present
in the gene, but do not show up).
Mendel wondered what happened to the recessive allele. To find out, he let the F1 plants self pollinate. He found
that some of the F2 plants had the recessive trait (short).
Mendel concluded the following: When each F1 plant flower and produces gametes (sex cells), the two alleles
separate from each other. As a result, each gamete carries only a single copy of each gene. Therefore, each F1
plant produces two types of gametes- those with the allele for tallness and those with the allele for shortness.
An easy way to determine possible gene combinations from a genetic cross is by drawing a diagram called a
Punnett Square. Make sure you fully understand how to use them by completing your handout!
Genotype: The genetic makeup of an organism. For instance, for tall pea plants, TT or Tt or tt.
Phenotype: Physical characteristics. TT and Tt plants have the same phenotype- they're both tall. But they don't
have the same genotype.
Heterozygous: Have two different alleles for the same trait. Tt is a heterozygous genotype.
Homozygous: Have the same alleles for the same trait. TT and tt are homozygous genotypes.
The next question Mendel had was whether one pair of alleles for one trait had any affect on another pair of
alleles for a completely different trait. For example, does a gene that determines whether a seed is round or
wrinkled have anything to do with the gene for seed color? To test this, he did a two factor (trait) cross for the F1
generation. All F1 were heterozygous for both alleles.
Next, he crossed the F1 generation. Would the 2 dominant alleles stay together? No- they indepently sorted,
showing that dominant alleles for one trait to not affect another trait. This is called independent assortment.
Genes are located on the chromosomes within the cell's nucleus. Mitosis is the division of the nucleus that
results in two identical daughter cells. Meiosis is the division of chromosomes that forms four cells (gametes),
each having half the number of the parent cell.
Meiosis: A process of reduction division in which the number of chromosomes per cell is cut in half through the
separation of homologous chromosomes in a diploid cell. This results in the production of four genetically
different haploid cells.
Homologous: Each chromosome has two parts, with each part coming from the opposite parent (1/2 from
mother, 1/2 from father)
Diploid: A cell that contains both (di) sets of homologous chromosomes
Haploid: A cell (gamete) that contains only one set of chromosomes. This cell is called sperm in males and egg
in females.
Chapter 12
Genes are made of DNA (Deoxyribonucleic acid), which stores and transmits the genetic information from one
generation to another. It is structured as a double helix- like a twisted ladder.
DNA is a long molecule made up of units called nucleotides. Each nucleotide is made up of a 5-carbon sugar, a
phosphate group, and a nitrogen containing base.
There are two groups of bases, with two bases in each group. Purines are Adenine (A) and Guanine (G).
Pyrimidines are Cytosine (C) and Thymine (T).
The sides of the helix are made up of sugar and phosphates. The bases make up the rungs of the ladder, and
are held together with hydrogen bonds. Bonds only form between certain bases pairs: adenine and thymine pair
together and guanine and cytosine pair together.
During DNA replication, the DNA separates into two strands. Each strand serves as a model for the new strand.
Following the rules of base pairing (A-T, G-C), new bases are added to each strand. The end result is two
identical strands.
The first step to copy DNA is RNA. The RNA molecule is different in 3 ways from DNA:
1. The sugar is ribose instead of deoxyribose.
2. RNA is single stranded.
3. RNA has uracil instead of thymine.
RNA is involved in making proteins. There are 3 main kinds:
1. Messenger RNA (mRNA)- has the instructions for joining amino acids to make a protein.
2. Ribosomal RNA (rRNA)- In ribosomes to help assemble proteins.
3. Transfer RNA (tRNA)- carries each amino acid to the ribosome according to the coded message in mRNA.
RNA is copied from DNA in a process called transcription, resulting in mRNA. During transcription, 3 things
happen.
1. The enzyme RNA polymerase binds to DNA and separates the strands.
2. RNA polymerase builds a strand of RNA using on strand of DNA as a template.
3. The DNA is transcribed into RNA using base pair rules, except that uracil binds to adenine.
The directions for making proteins are in the order of the four nitrogenous bases. This code is read 3 letters at a
time. Each codon, group of 3 nucelotides, stands for an amino acid. One codon signals the start of protein
building, one signals the end. When all of the codons for a particular protein have been read, the resulting amino
acids form the completed protein. This process is called translation.
Before translation begins, mRNA has to be transcribed from DNA. Then, the newly formed mRNA moves into the
cytoplasm and attaches to a ribosome. As each codon of the mRNA moves through the ribosome, the proper
amino acid is brought to the ribosome by tRNA. This grows the protein chain until the stop codon is reached. The
newly formed protein is released and translation is complete.
Mutations are mistakes made when a cell copies its own DNA. This mistake changes the genetic material of the
cell, causing a change in the blueprint.
Two main types of mutations:
1. Gene mutations- changes in a single gene.
a) Point mutation- one base pair replaces another base pair. Only affects the making of one amino acid.
b) Frameshift mutation- an addition or deletion of a nucleotide, causing the entire reading frame to shift,
causing all amino acids to be "wrong", and producing a completely different protein.
2. Chromosomal mutations- a change in the number or structure of chromosomes. Down's syndrome is a result
of an extra chromosome on chromosome 21.