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3.03: Genetics , Punnett squares. Mendel. Gregor Mendel – a monk who became known as the “father” of genetics. He grew and studied pea plants in the monastery where he lived. Mendel used pea plants because peas: Are easy to grow . Produce a lot of offspring . Mature quickly.

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  • Gregor Mendel – a monk who became known as the “father” of genetics. He grew and studied pea plants in the monastery where he lived. Mendel used pea plants because peas:
    • Are easy to grow.
    • Produce a lot of offspring.
    • Mature quickly.
important genetics vocabulary
Important Genetics Vocabulary:
  • Trait – a characteristic, Ex: plant size, seed color, pod shape
  • Genetics – the study of how traits are passed from one generation to the next.
  • Gene – the factors that control traits (found in the DNA). In a pea plant, it has a gene that controls its color, another for its size, and another for shape.
  • Allele – different forms of a gene. Each allele is represented by a letter. T = tall, t = short
  • Homozygous – organism with two identical alleles for the same trait (TT or tt).
  • Heterozygous – organism with two different alleles for the same trait (Tt)
  • Dominant – allele that is expressed, even in the presence of a recessive allele. The dominant allele is always capitalized. TT or Tt = tall
  • Recessive – allele that is expressed only when homozygous. The recessive allele is always lower-case. tt = short
  • Phenotype – physical characteristics, what the traits look like (tall, brown)
  • Genotype – the genetic makeup, the letters of the alleles. The capital letter is always written first. (TT, Tt, Hh)
mendel s principles of inheritance
Mendel’s principles of inheritance:
  • Inherited traits are transmitted by genes which occur in alternate forms called alleles.
  • Principle of Dominance – when 2 forms of the same gene are present, the dominant allele is expressed and masks the recessive allele.
  • Principle of Segregation – in meiosis two alleles separate so that each gamete receives only one form of the gene.
  • Principle of Independent Assortment – each trait is inherited independent of other traits (chance)
predicting inheritance
Predicting inheritance
  • How can we determine what the offspring are going to be?
  • Punnett square – chart showing the possible combinations of alleles in a cross.
  • Punnett squares show the probability of getting a certain type of offspring.
  • Time for the chalk board!
test cross
Test Cross
  • If we saw an organism that had round seeds, and round is dominant, how could we figure out what its genotype is?
  • Test Cross– a cross between an unknown and a homozygous recessive. Ex: In pea plants, round seeds are dominant to wrinkled seeds. So:
  • Round seeds = either RR or Rr. Cross it with a wrinkled seed plant. Wrinkled = rr
  • If any of the offspring have wrinkled seeds, then the unknown must have been Rr.
non mendelian genetics1
Non-mendelian Genetics
  • The type of inheritance we learned about before was called complete dominance. One trait was completely dominant over the other, like tall is dominant to short, with no in-between. We only used one letter to stand for the trait, with upper case for the dominant and lower case for the recessive.
non mendelian genetics2
Non-mendelian Genetics
  • These are called Mendelian traits because this is the type of traits studies by Mendel when he worked with pea plants.
  • Today we will learn about non-Mendelian traits. For these, we will use two capital letters to stand for the different alleles, and both letters to stand for the hybrid (heterozygote).
non mendelian genetics3
Non-mendelian Genetics
  • Incomplete Dominance – neither allele is completely dominant over the other, the combination of alleles creates a new phenotype that is a blend of the others.
  • For example, a certain plant shows incomplete dominance. It can have red flowers, white flowers, or pink flowers. Let’s assign letters to the alleles:

RR = red, WW = white,

RW = pink

  • Make a Punnett square showing a cross between a red flowered plant and a pink flowered plant:
non mendelian genetics4
Non-mendelian Genetics
  • Codominance – both alleles are dominant and both will show in a hybrid (heterozygote).
  • In guinea pigs, black and white fur colors are codominant. A hybrid will show black and white spots. Let’s assign letters to the alleles:

BB = black fur, WW = white fur,

BW = black and white spotted fur

  • Make a Punnett square showing a cross between a white guinea pig and a spotted guinea pig:
non mendelian genetics5
Non-mendelian Genetics
  • Polygenic traits are traits that are influenced by several genes. Examples include human height and skin color, which are both influenced by dozens of genes.
  • As a result of polygenic traits, a lot of intermediate conditions exist.
blood types
Blood types
  • Genes with 3 or more alleles are said to have multiple alleles. An example of multiple alleles is human blood type.
  • The four blood types are A, B, AB, and O. A and B are codominant and O is recessive. They are sometimes written with the letter I: IA, IB, i
  • Since O is recessive, this means that if A is with O, A will be the blood type. If B is with O, then B will be the blood type. But if A is with B, then the blood type is AB.
blood types2
Blood types
  • What blood types can be donated? It depends on the blood types of the receiver and the donor.
    • Type A can give to A or AB, because both contain A
    • Type B can give to B or AB, because both contain B
    • Type AB can give to AB only, because only AB contains both A and B
    • Type O can give to all blood types since it is recessive.
    • AB is considered the universal acceptor, O is the universal donor.
blood types3
Blood types
  • Make a Punnett Square showing a cross between a woman with Type A homozygous blood and a man with Type B heterozygous blood:
  • Make a Punnett Square showing a cross between a woman with Type A heterozygous blood and a man with Type O blood:
sex linked traits1
Sex-Linked Traits
  • Humans have 46 chromosomes, 23 pairs, in their cells. 22 of those pairs are called autosomal chromosomes. The last pair are the sex chromosomes:
  • Women have XX and men have XY sex chromosomes.
sex linked traits2
Sex-Linked Traits
  • If a gene is autosomal, it will appear in both sexes equally, because it is found on an autosomal chromosome. Everyone has the same 22 pairs of autosomal chromosomes.
  • If a gene is sex-linked, it is located on a sex chromosome. Sex-linked genes are usually found on the X chromosome and are usually recessive.
sex linked traits3
Sex-Linked Traits
  • Sex-linked traits are more often seen in males than females. Because males are XY, they only have one X chromosome, and therefore a male with oneaffected X will show the trait.
  • Females, on the other hand, need 2 of the recessive alleles in order to show the trait, because they have two X chromosomes. Therefore, many females act as carriers of the genes and show the trait less often.
  • A carrier is a female that is heterozygous for a sex-linked trait. She carries the gene and can pass it on to her offspring, but does not show the trait herself.
sex linked traits4
Sex-Linked Traits
  • Two examples of common sex-linked disorders are:
  • 1. Color-blindness—most common is red/green colorblindness: people can’t tell the difference between red and green when they are placed side by side.
sex linked traits5
Sex-linked Traits
  • 2. Hemophilia—a disease where your blood does not clot. Even a small cut or bruise could kill!
sex linked traits6
Sex-Linked Traits
  • Hemophilia (and all other X sex-linked traits) only occur when all of the X chromosomes have a copy of the recessive gene.

H = normal, h = hemophilia

XHXH : normal female

XHXh : female carrier

XhXh : female hemophiliac

XHY : normal male

XhY : male hemophiliac

sex linked traits7
Sex-linked traits
  • Let’s practice some sex-linked trait problems. The first step when doing a sex-linked trait problem is to write in all the X’s and Y’s first, then label them with the alleles.
  • The risks of passing on a genetic disorder to offspring can be assessed by genetic counseling, prenatal testing, and by analyzing a pedigree.
  • A pedigree is a family history diagram that shows how a trait is inherited over several generations.
  • A pedigree can be mapped out to determine if individuals are carriers or if their children might inherit the disorder.
  • Carriers are individuals who are heterozygous for an inherited disorder but do not show symptoms. Carriers can then pass the allele for the disorder on to their children.
  • In a pedigree, females are indicated by circles, males are indicated by squares.
  • Shaded figures represent individuals with the trait, a carrier could be 1/2 shaded (but not always!).
  • Generations are numbered with roman numerals (I, II, III, IV) from top to bottom.
  • People within generations are numbered (1, 2, 3) from left to right.
  • People that are married (or just having children together) are connected by a horizontal (left to right) line. Offspring of individuals are connected to their parents by a vertical (up and down) line.
  • By analyzing a pedigree you can tell if a trait is dominant or recessive and if it is autosomal or sex-linked.

One parent has the disease, and none of the three children inherited it. We can tell that this is a recessive trait because not many people in the family have it. If it were a dominant trait, many more would have inherited it.

Both males and females show the trait, so we know this is not sex-linked but is an autosomal trait.

  • We can also analyze a pedigree to figure out people’s genotypes. If we know this is an autosomal recessive trait, then anyone shaded in must have the genotype (nn). Anyone not shaded must have either (NN or Nn).
  • What is the genotype of person I1?


  • Person IV2?

Nn or NN

  • Who in this pedigree must be heterozygous?

III1 and III2

  • Only women are carriers, and only men show the trait. Therefore, this must be a sex-linked trait.
  • We can also tell this is a recessive trait, because not many people have it. In order for a trait to have carriers, it must be recessive. If a trait is dominant, people either have it or they don’t.
  • Since this is a sex-linked recessive trait, what is the genotype of Alice?


  • What is the genotype of Fred?


  • Genetic disorders may be detected by using prenatal testing and pedigrees. They can also be detected using karyotypes.
  • A karyotype is a photograph of an individual’s chromosomes in a dividing cell during mitosis. The chromosomes are arranged by size and numbered.
  • A karyotype can show you two things:
  • Chromosome abnormalities: missing chromosomes, extra chromosomes, or if chromosomes are malformed
  • The sex of the person

Normal Karyotype

Down’s Syndrome

genetic disorders
Genetic disorders

Down’s Syndrome:

  • A chromosomal disorder caused by an extra chromosome 21. For this reason it is also known as Trisomy 21 (which means 3 chromosome 21’s).
  • Caused by nondisjunction, which means that during meiosis a gamete is produced with an extra copy of chromosome 21. This is not an inherited trait, it happens in the egg or the sperm before fertilization.
  • Symptoms: learning disabilities, developmental disabilities, and impaired physical growth.
  • Occurrence: about 1 of every 9,000 births.
genetic disorders2
Genetic disorders

Cystic Fibrosis:

  • Symptoms: causes thick mucus to coat the lungs leading to severe breathing problems. It also causes the pancreas to not secrete enzymes as efficiently as it should, causing poor growth, diarrhea, and vitamin deficiency.
  • Inheritance: autosomal recessive disease caused by a mutation in a gene.
  • Occurrence: 1 in 3,900 children are born with this disease, and there is no cure.
genetic disorders4
Genetic disorders

Huntington’s Disease:

  • Symptoms: a genetic neurological disorder characterized after onset by uncoordinated, jerky body movements and a decline in some mental abilities. People with Huntington’s Disease have too many CAG’s in a gene on their DNA and so form a mutant protein from too many glutamines.
  • Occurrence: Up to 7 people in 100,000 have this disorder.
  • Inheritance: This is an autosomaldominant trait, so an affected individual needs just one copy of the gene to show the disease.
genetic disorders6
Genetic disorders

Sickle Cell Anemia:

  • A blood disorder in which the red blood cells are not flexible and round but are rigid and sickle-shaped (like a crescent moon). This restricts the blood cells’ movement throughout the blood stream and decreases the amount of oxygen the cells can carry through the body.
  • Inheritance: a recessive trait.
  • Symptoms: misshapen blood cells cause the blood to not carry enough oxygen throughout the body. Individuals most often feel fine, but their lives are interrupted by periodic painful attacks. The only treatment is pain medication during these attacks.
  • Occurrence: 1 out of every 10 African-Americans has this trait.