By conducting careful experiments with plants, Mendel discovered how
He isolated color a trait with only 2 variants: yellow and green.
He also isolated texture: wrinkled and smooth
“true bred” them Crosses with green and green always produced green and yellow with yellow
always made yellow.
F0gen original (1g,1y)
F1gen the offspring (all y)
F2gen offspring of f1, (3 yellow for every 1 green) 3:1
He formulated 2 principles:
1. The observational characteristics of organisms are determined jointly by two particles, one
inherited from the mother and one from the father.
T.H. Morgan called these ‘genes’
2. Each of these 2 particles or genes, is equally likely to be transmitted when gametes (egg and
sperm) are formed. Modern scientists call this ‘independent assortment’.
Cell division and the role of chromosomes in inheritance:
Nobody paid any attention to Mendel’s results for almost 40 years.
When Mendel’s results were rediscovered, they were widely accepted because
scientists now understood the role of chromosomes in the formation of gametes. The chromosome was discovered. Humans have 23 pairs.
Mitosis and Meiosis:
Every cell has a nucleus.
The process of ordinary cell division mitosis
Different organisms have different numbers of chromosomes, but in diploid organisms,
chromosomes are in homologous pairs (similar shapes and staining methods).
All primates are diploid.
Mitosis 2 features suggest chromosomes play an important role in determining the properties of
1. the original set of chromosomes is duplicated so that each daughter cell has an exact copy of the
chromosomes present in the parent.
2. the material that makes up the chromosome is present even when the cell is not dividing. (most
of the time they are in resting period, where you cannot see the info. It is still there)
each cell contains a pair of homologous chromosomes
in meiosis, the special cell division process that produces gametes, only half of
the chromosomes are transmitted from the parent cell to the gametes.
Meiosis makes gametes
only has one copy of each chromosome. – haploid
haploid sperm + haploid egg= diploid zygote.
Chromosomes and mendel’s experimental results: Mendels principles can be deducted from the assumption that genes are carried
on two chromosomes.
Walter Sutton connected chromosomes and inheritance.
Different varieties of a particular gene are called “alleles”. Individuals with two
copies of the same allele are “homozygous”. Individuals with different alleles are
Gene material particles carried on a gene
Allele different varieties of a single gene
A cross between a homozygous dominant parent and a homozygous recessive
parent produces all heterozygotes in the F1 generation
Genotype the particular combination of genes or alleles that an individual carries.
Phenotype the observable characteristics of the organism, such as the pea color.
The A allele is dominant because individuals with only one copy of the dominant allele have the
same phenotype as individuals that have two copies.
The a allele is recessive because it has no effect on phenotype in heterozygotes.
A cross between heterozygous parents produces a predictable mixture of all
1:2:1 ratio. ¾ will be yellow.
25% chance AA
25+25=50% chance of Aa
25% chance aa Can also use a Punnett square
Linkage and Recombination:
Mendel also performed experiments involving two traits that he believed showed
that separate characters segregate independently.
Cross of smooth yellow and wrinkled green AABB and aabb
F1= all smooth yellow AaBb
F2= 9 smoothyellow (AB), 3 smooth green(Ab), 3 wrinkled yellow(aB), 1 wrinkled green (ab)
Shows that sexual reproduction produces a new combination od traits called recombination.
Independent segregation occurs mainly when the traits measured are controlled by genes that
reside on different chromosomes
Genes are arranged on chromosomes like beads on a string.
Locus (pl. loci) site on a chromosome where a particular gene is found
One locus can hold multiple alleles
All of the genes carried on all of the chromosomes are referred to as the genome. Loci for different traits occur on the same chromosome linked
Different chromosomes unlinked.
Since chromosomes tangle and break during meiosis, they are not always preserved intact. Genes
on one chromosome can be shifted from one member of a homologous pair to another. This is the
process of crossing over.
The rate at which recombination generates novel combinations of genes at two loci on the same
chromosome depends on the likelihood that a crossing over event will occur.
If 2 loci are close together, crossing over = rare, recombination – low
If far, crossing over = common, recombination the rate for genes on different chromosomes.
Genes are segments of a long molecule called DNA which is contained in
Watson and Crick
Understanding the chemical nature of the gene is critical to the study of human
evolution. 1) molecular genetic links biology to chemistry and physics, and 2)
molecular methods help us reconstruct the evolutionary history of the human
Shows us that the last common ancestor of humans and chimps was 57 mya.
Genes are DNA:
DNA is usually well suited to be the chemical basis of inheritance.
Each strand has a backbone of sugar and phosphate molecules. Attached to the sugar is one of 4
bases: adenine, guanine, cytosine or thymine. The 2 strands of dna are held together by hydrogen bonds.
TA CG –base pairs
Every single dna molecule is stable chemically. It must be stable and be able to be replicated.
The message encoded in DNA affects phenotypes in several different ways.
1. DNA in protein coding genes specifies the structure of proteins. Many proteins are enzymes
regulate the body’s biochemical machinery
2. DNA in regulatory genes determines the conditions under which the message encoded in a
protein will be expressed. Plays a crucial role in shaping the differentiation of cells during
3. Other DNA specifies the structure of different kinds of RNA molecules which produce important
cellular functions. Includes protein synthesis.
Some Gene Code for Proteins:
proteins called enzymes influence an organisms biochemistry.
They determine what the raw materials are transformed into when cells are built.
Proteins play a number of other important roles in the machinery of life.
Ligaments and tendons collagen
Some are tiny valves that regulate what goes into an