Download The Science of Heredity

The Science of Heredity
The body is made
up of trillions of cells
Chromosomes are found
inside the nucleus of every cell
Each chromosome
is made up of DNA
consisting of many
genes that control
our hereditary traits
(Your teacher will give you a copy
of this handout to keep.)
180. Vocabulary list for “Cells & Heredity”
181. Gregor Mendel
• A priest from central
Europe in 1800’s
• Grew pea plants in the
monastery garden
• Noticed that plants were
often similar to their
parents, but other times
the seeds produced
different traits (physical
characteristics) in the
offspring plants
182. Mendel’s Experiments
• Mendel experimented with thousands of pea
plants for over ten years to find out HOW
traits are passed on
• The passing of traits from parents to offspring
is called heredity
• Mendel’s work formed the foundation of
genetics which is the scientific study of
heredity
183. Crossing Pea Plants
• Peas were a good choice because they have
many traits that exist in only two forms each
– Ex: tall or short plants; green or yellow seeds
• Pea plants produce large amounts of offspring
• The petals of pea flowers enclose the
male & female parts, so that they
self-pollinate; Mendel could then
choose which plants to cross-pollinate
by hand in his experiments.
How Mendel Cross-Pollinated Peas
184. Mendel’s Experiments
• Started his experiments with purebred plants
• A purebred organism is one that always
produces offspring with the same traits as the
parent
– Ex: purebred short plant always produces more
short offspring
185. First Experiment
Parental Generation: (P = parents)
He crossed purebred tall with purebred short
Seeds grew into First Filial Generation (F1 = children):
All F1 were tall
Then, he allowed F1 to self-pollinate
Seeds now grew into Second Filial Generation (F2 = grandchildren)
In F2 75% were tall, and 25% were short
First Experiment Looks Like This:
186. Genes
• Mendel reasoned that every trait must be
controlled by two factors of inheritance
• One factor is from the male parent and one
from the female parent
• Factors that control traits are known today as
genes
• Different versions of a gene are called alleles
– Ex: Gene for height has two alleles tall or short
187. Allele Expression:
• A dominant allele is one whose trait always
shows up in the organism when the allele is
present; it is like the “default setting”
• Ex: If your dad gives you a tall (dominant)
gene, and mom gives you a short (recessive),
you will still be tall
• A recessive allele can ONLY show up if the
dominant one is missing
• Ex: The only way you can be short, is if both
parents gave you a short gene
Dominant & Recessive Pea Genes
188. Gene Symbols
• Capital letter represents a dominant allele
• A lowercase letter represents a recessive allele
• Ex: The allele for tall stems is T, and the allele for
short stems is t
Symbol Practice
What type of offspring (tall or short?) would grow
from the seed if the following alleles were
combined:
• TT = ___________ plant
• Tt = ___________ plant
• tt = ____________ plant
(This page is a handout your teacher will give you)
189. Understanding Mendel’s Crosses
Genes
Purebred X Purebred --> Hybrid X Hybrid -->
¾ tall (75%) and
¼ (25%) short
190. Probability
• What are the chances of a coin coming up
“heads” when you toss it? One side comes up
each time, out of two possible sides, so that’s
½ = 50%
• Probability is the likelihood that a particular
event will occur based on mathematics
191. Mendel and Probability
• When Mendel crossed hybrids (Tt) his results
were consistent with probability’s prediction:
¾ of the plants were tall and ¼ were short
• He was the first scientist to realize
mathematical probability could be used to
predict the outcome of genetic crosses
Mendel used his skill in math to calculate
probabilities:
• A female plant that is Tt will shuffle these alleles
into its eggs, and ½ will get a T while the other ½
of the eggs will get a t. The total eggs = (½ T + ½ t)
• A male plant that is Tt will shuffle these alleles
into its sperm, and ½ will get a T while the other ½
of the sperm will get a t. Total sperm= (½ T + ½ t)
• Then, when the eggs and sperm meet during
fertilization: (½ T + ½ t) X (½ T + ½ t)
= ¼ TT + ¼ Tt + ¼ Tt + ¼ tt
= ¼ TT
+ ½ Tt + ¼ tt
¾ tall
+ ¼ short
192. Punnett Squares – created by
a scientist who came after Mendel
• A punnett square is a grid that shows all the
possible combinations of alleles that can
result from a genetic cross without using math
Ex: cross a Tt plant x Tt plant
• The cross of hybrids from the last slide
produces offspring that look like this:
• Ex: cross RR x rr in this punnett square
• Ex: cross Rr x rr in this punnett square
193. Phenotype and Genotype
• An organism’s phenotype is its physical
appearance or what it looks like (tall)
• An organism’s genotype is its genetic makeup
shown by the allele combination of letters (TT)
194. Homozygous vs. Heterozygous
(Alike)
(Different)
• An organism that has two identical alleles is
said to be homozygous (bb or BB)
• An organism that has two different alleles is
said to be heterozygous (Bb)
Practice
• Determine if the following are homozygous or
heterozygous:
TT __________ Ss_________
Yy__________
Bb__________ aa_________
MM_________
Practice
• Determine the phenotype (physical appearance) of
the following:
Yellow peas are dominant to green peas
YY_________
Yy__________ yy__________
Practice
• For each phenotype give the possible
genotypes (letter combinations)
– A tall(T) plant is dominant to a short(t) plant
Tall__________
short__________
Animated Tutorial on Punnett
Squares:
• http://www.sumanasinc.com/webcontent/ani
mations/content/mendel/mendel.html
Genetics Tutorial
• http://learn.genetics.utah.edu/content/begin/
tour/