Transcript Genetics - msdemarco

Genetics
Genetics
• Genetics is the study of heredity.
• Heredity is when a parent passes down
physical characteristics to their offspring.
• These different physical characteristics are
known as traits.
Mendel’s Experiment
• Gregor Mendel’s experiments helped create
the field of genetics.
• Mendel studied the characteristics of pea
plants and wondered why they had different
characteristics. What determined which
characteristic the pea plant would exhibit?
Mendel’s Experiment
• Pea plants usually self-pollinate, meaning that
they are the only parent to their offspring.
• Mendel cross-pollinated the pea plants by
removing the pollen from one plant and
brushing on the pollen from a second pea
plant.
Mendel’s Experiment
• Mendel crossed pea plants that showed
different traits.
• He used purebred pea plants to start his
experiment.
– A purebred organism is one that has exhibited the
same trait for many generations.
Mendel’s Experiment
• The P generation
– The P generation is the parent generation and
they are the original plants.
– In Mendel’s experiment his P generation had one
purebred tall pea plant and one purebred short
pea plant.
Mendel’s Experiment
• The F1 Generation
– The offspring of the P generation is known as the
F1 generation, or the first filial generation.
• The word filial comes from the Latin words for
“daughter” and “son”.
– In Mendel’s experiment, all of the offspring in the
F1 generation was tall, despite one parent being
short.
– The shortness trait seemed to have disappeared.
Mendel’s Experiment
• The F2 Generation
– Mendel allowed the F1 generation to selfpollinate.
– This new offspring Mendel called the F2
generation.
– Somehow, the shortness trait seemed to have
reappeared.
– About ¾ of the F2 generation was tall, while ¼ was
short.
Mendel’s Experiment
• Mendel also crossed pea plants with different
contrasting traits.
• In each of Mendel’s crosses, only one trait
appeared in the F1 generation.
• However, each time, the “lost” trait appeared
in about ¼ of the F2 generation.
Alleles
• Today, scientists use the word gene for the
factors that control a trait.
• They refer to the different forms of that trait
as alleles.
Alleles
• An organism’s traits are controlled by the
alleles it inherits from its parents.
– It receives one allele from each.
• Some alleles are dominant.
• Some are recessive.
Alleles
• A dominate trait will always show up if an
allele for it is present in an organism.
• A recessive trait only appears if both alleles
are recessive.
Alleles
• If an organism has one of each type of allele it
is considered a hybrid.
• If an organism has both a dominant and a
recessive allele, it will only show the dominant
trait.
– For example, tall height was dominant, while short
was recessive in the pea plants.
Alleles
• Scientists represent dominant alleles with
capital letters and recessive alleles with
lowercase letters.
Probability
• Probability is a number that describes how
likely it is that an event will occur.
• When you have two options that are equally
likely to occur, there is a 50% chance that
either will happen.
Probability
• The laws of probability predict what is likely to
occur, not what necessarily will occur.
• Because of this, the results of an event, such
as flipping a coin, will not always be exactly
50/50.
• The more times you try (for example, flipping
the coin) the closer to 50/50 your results will
be.
Probability
• The results of the first flip of a coin does not
affect the results of the next flip.
Punnett Squares
• Scientists use Punnett squares to help show
how probability applies to genetics.
• Punnett squares show all the possible
combinations of alleles that can happen after
a genetic cross.
– A genetic cross is when two parents produce an
offspring and each pass down one allele.
Punnett Squares
Punnett Squares
• You can use a Punnett square to calculate the
probability that an offspring will have a certain
combination of alleles.
• In a genetic cross, the allele that each parent
will pass on to its offspring is based on
probability.
Phenotypes and Genotypes
• An organism’s phenotype is its physical
appearance, or visual traits.
• An organism’s genotype is its genetic makeup,
or allele combinations.
Phenotypes and Genotypes
• Two organisms can show the same dominant
phenotype, but have different genotypes.
– One can have two dominant alleles.
• Example: RR
– The other can have one dominant and one
recessive allele.
• Example: Rr
Phenotype and Genotype
• An organism that has two identical alleles for
a trait is said to be homozygous. (RR or rr)
• An organism that has two different alleles for
a trait is said to be heterozygous. (Rr)
• We know that homo means “the same” and
hetero means “different”.
Phenotype and Genotype
• Mendel used the term hybrid to describe
heterozygous pea plants.
Codominance
• For all of the traits that Mendel studied, one
allele was dominant while the other was
recessive.
• This is not always the case.
Codominance
• Codominance is when the alleles are neither
recessive nor dominant.
• As a result, when these alleles are combined,
both show up in the offspring’s appearance.
The Chromosome Theory of
Inheritance
• According to the chromosome theory of
inheritance, genes are carried from the
parents to their offspring on chromosomes.
The Chromosome Theory of
Inheritance
• Sex cells (sperm and egg) have exactly half the
number of chromosomes found in an
organism’s body cells.
• When the sperm and the egg join during
fertilization, the offspring ends up with exactly
the same number of chromosomes in its cells
as each of its parents have in theirs.
The Chromosome Theory of
Inheritance
• One allele for each gene comes from the
organism’s female parent and the other allele
comes from the male parent.
• These paired alleles were carried on paired
chromosomes.
Meiosis
• Meiosis is the process by which the number of
chromosomes is reduced by half to form sex
cells.
Meiosis
• Before meiosis, every chromosome in the cell
is copied.
• Then, these pairs line up in the center of the
cell, separate, and move to opposite ends of
the cell.
• The cell splits, creating two cells, each with
half of the chromosomes.
Meiosis
• These chromosomes now move to the center
of the new cells.
• The centromeres split, and the chromatids
move to opposite sides of the cells.
• These cells split, creating sex cells with only
one chromosome from the original pair.
Human Chromosomes
• Humans have 23 chromosome pairs (46 total
chromosomes) in every cell.
• Chromosomes are made up of many genes
joined together.
• Although you have 23 chromosome pairs, you
have about 35,000 genes.
Human Chromosomes
• One chromosome in each chromosome pair
comes from the mother and the other from
the father.
• Each chromosome in the pair has the same
genes, but the alleles for the gene in the two
chromosomes can be different.