Genetics - 7th Grade
Essential Vocabulary Words:
allele, Chromosome Theory of Inheritance, chromosome, DNA, dominant, gene, genotype, heterozygous,
homozygous, nucleus, phenotype, Punnett Square, recessive, traits
Content Information:
Inside every cell, the smallest unit of life, there is genetic material called Deoxyribonucleic Acid or DNA. This
DNA is a network of nitrogen bases, phosphates, and sugars arranged in double helix pattern that looks like a spiral
ladder. DNA creates instructions for all of the cell functions and is found in the nucleus of eukaryotic cells. DNA is the
genetic code that is known as the blueprint or recipe for making structures. DNA is found in chromosomes.
Chromosomes are strands of DNA that contain genes.
Genes are what organisms inherit from parents. According to the Chromosome Theory of Inheritance, genes
are passed from parents to offspring on chromosomes. The genes on chromosomes have control for different traits. For
example a gene may control eye color. All genes come in pairs. Different forms of genes are known as alleles. For
example, when considering eye color in human beings, the color is the gene. The specific color of brown eyes, blue eyes,
and green eyes is the allele. The genes on chromosomes control different traits. For example, a gene may control eye
color. All genes come in pairs.
Dominant allele – a trait that always shows up when the allele is present
– Symbol – Capital letter (T)
Recessive allele – a trait that is covered up or masked when a dominant allele is present
– Symbol – lower case letter (t)
For example, brown eyes (B) are dominant over blue (b), and appear more often.
B represents the dominant trait brown; b represents the recessive trait of blue.
The child could have B from one parent and B from the other = child with brown eyes
GENOTYPE: BB
PHENOTYPE: Brown Eyes
The child could have B from one parent and b from the other = child with brown eyes.
GENOTYPE: Bb
PHENOTYPE: Brown Eyes
The child could have b from one parent and b from the other = child with blue eyes
GENOTYPE: bb
PHENOTYPE: Blue Eyes
Any time the dominant gene appears, the offspring will have the dominant trait.
Genotype
Phenotype
BB or Bb
Brown Eyes
bb
Blue Eyes
Humans have 46 chromosomes in each body cell or 23 pairs. The sex chromosomes are on the 23rd
pair. This combination of alleles determines the sex of the offspring, XX for female and XY for male. All of
the other pairs of chromosomes are called autosomes. Some traits are carried down from parent to offspring on
the X or Y chromosomes. For example, colorblindness is a trait controlled by a recessive allele on the X
chromosome. Since males only have one X in their genotype, more males have colorblindness than females. If
the recessive trait for colorblindness is present in the X allele, the trait will be present.
Pea Plants have 14 chromosomes in each cell or 7pairs. We are made of thousands of genes (over 60,000
in humans). Each parent contributes half of each chromosome pair to their offspring. Usually there is one pair
of sex chromosomes. Offspring receive one member of each pair from their mother, the other member of each
pair from their father.
Humans: 23 chromosomes from mother
23 chromosomes from father
(XX)
(XY)
Pea Plants: 7 chromosomes from mother (XX)
7 chromosomes from father (XY)
Genotype is the combination of alleles and phenotype is the physical trait that shows up as a result of the allele
combination or genotype. There are three possible genotypes or combination of alleles. When there are two of
the same allele, the genotype is considered homozygous or purebred. For example, BB and bb are
homozygous or purebred genotypes. When two different alleles make up a genotype, the pair is considered
heterozygous or hybrid. For example, Bb is a heterozygous or hybrid genotype.
Gregor Mendel
Gregor Mendel was a monk that experimented with cross pollinating pea plants. In his findings, he discovered
a pattern in genetic probability. He cross pollinated a tall homozygous/purebred pea plants with short
homozygous/purebred pea plants. The pea plants that resulted in the cross pollination were all tall (F1
Generation). These tall pea plants self-pollinated. The result of the self-pollination (F2 Generation) was that
75% of the pea plants were tall and 25% of the pea plants were short. Because of this finding, the short trait
was proven to be a recessive trait. The illustration below provides an illustration of Mendel’s experiment.
Image courtesy of http://library.thinkquest.org/17109/tutorial.htm
Gregor Mendel Experimented with several traits in pea plants
 shape of the seed: round or wrinkled.
 color of the pea: yellow or green.
 color of the seed: gray or white.
 form of the ripe pod: inflated or constricted between peas.
 color of the unripe pod: green or yellow.
 position of the flower: terminal or axial.
 length of the stem: tall or short
Mendel’s genetic probability can be expressed using a Punnett Square developed by English geneticist
Reginald Punnett in the early 20th century.
T= tall/dominant
t= short/recessive
Parents: Tt and Tt
Genotypes
TT
Tt
Tt
tt
Phenotypes
Tall
Tall
Tall
short
T
t
T
t
TT
Tt
Tt
tt
Results:
3 out 4 or 75% of the dominant trait or tall trait is shown.
1 out 4 or 25% of the recessive trait or short trait is shown.
A common misconception about dominant and recessive traits is that dominant traits show up in
organisms more so than recessive traits. Ideally, since 2 out of 3 genotypes produce a phenotype that shows the
dominant trait, this thinking makes sense but is not the case. Some students may see this 2 to 3 ratio and use it
to support their hypothesis. It is important that you highlight factors that affect an organism’s percentage of
dominant to recessive traits. These factors include the frequency of the various alleles in the gene pool and the
probability (not certainty) of traits being passed on.
A certain population of organisms may reproduce in an environment where a recessive trait is more
suited to that environment over the dominant trait for survival. In this instance, the recessive trait becomes
more prevalent in that population of organisms and is more likely to occur. In the genoworm lab, your results
for each pair and total class results may show more recessive traits or may show more dominant traits. These
results can occur in the Human Genetics Population Study as well. A common conclusion you may draw with
your students is that the dominant trait will not always show up more in a population based on the factors
mentioned.
GENETICS REVIEW
BACKGROUND INFORMATION
Inside every cell, the smallest unit of life, there is genetic material called deoxyribonucleic acid or
________. This DNA is a network of nitrogen bases, phosphates, and sugars arranged in double helix pattern.
This looks like a spiral ladder. DNA creates instructions for all of the cell functions and is found in the
___________ of eukaryotic cells. DNA is the genetic code that is known as the blueprint or recipe for making
structures. DNA is found in chromosomes. Chromosomes are strands of DNA that contain genes.
__________ are what organisms inherit from parents. According to the
_________________________________, genes are passed from parents to offspring on chromosomes. The
genes on chromosomes control different traits. For example, a gene may control eye color. All genes come in
pairs. There are different forms of genes for a certain trait. Different forms of genes are known as __________.
For example, when considering eye color in human beings, the color is the gene. The specific color of brown
eyes, blue eyes, and green eyes is the allele. The genes on chromosomes have control for different traits. For
example a gene may control for eye color. All genes come in pairs.
Human Genetics Population Study
Have you ever wondered why some people have blue eyes while others have brown eyes? Why do some
people have a widow’s peak, while others do not display this hairy trait? The answer is in our genes. Genes
have different forms called alleles that determine which traits we will inherit from our parents. The combination
of alleles will determine what an organism looks like. The physical characteristic an organism displays is
known as the organism’s phenotype. The combination of the alleles and the actual genetic make-up of the
organism is known as the organism’s genotype. A dominant allele is one that always shows up in the
organism. A recessive allele is an allele that is masked or hidden when a dominant allele is present.
Meiosis
Definition
Function
Type of
Reproduction
Occurs in
Genetically
Crossing Over
Pairing of
Homologous
Number of Divisions
Number of Haploid
Daughter Cells
Creates
Chromosome
Number
Mitosis
A type of cellular reproduction in
which the number of chromosomes
are reduced by half through the
separation of homologous
chromosomes, producing two
haploid cells.
A process of asexual reproduction
in which the cell divides in two
producing a replica, with an equal
number of chromosomes in each
resulting diploid cell.
Sexual Reproduction
Cellular reproduction & general
growth & repair of the body
Asexual
Sexual
Humans, animals, plants, fungi
Different
Yes, mixing of chromosomes
can occur
Yes
All organisms
identical
No, crossing over cannot occur
2
4
1
2
Sex cells only- female egg cells
or male sperm cells
Reduced by half
Makes everything other than
sex cells
Remains the same
No
Website Links:
Dominant VS Recessive
http://www.thetech.org/genetics/ask.php?id=188
http://www.blinn.edu/socialscience/ldthomas/feldman/handouts/0203hand.htm
Gregor Mendel
http://anthro.palomar.edu/mendel/mendel_1.htm
http://anthro.palomar.edu/mendel/glossary.htm#genetics
http://library.thinkquest.org/17109/tutorial.htm
Punnett Square Practice
http://biology.clc.uc.edu/courses/bio105/geneprob.htm
http://www2.edc.org/weblabs/Punnett/punnettsquares.html
http://www.hobart.k12.in.us/jkousen/Biology/psquare.htm
http://ethemes.missouri.edu/themes/1015
Dragon Lab
http://www2.edc.org/weblabs/DragonMeiosis/DragonMeiosisMenu.html
Genetics Web Labs
http://www2.edc.org/weblabs/WebLabDirectory1.html
Genetics Games
http://nature.ca/genome/04/041/041_e.cfm
DNA: The Double Helix
http://nobelprize.org/educational/medicine/dna_double_helix/
DNA Detective
http://www.tryscience.org/experiments/experiments_dna_online.html
Blood Typing Game
http://nobelprize.org/educational/medicine/landsteiner/readmore.html
CSI Experience
http://forensics.rice.edu/
Interactive DNA
http://www.pbs.org/wgbh/nova/sheppard/analyze.html