Gregor Mendel used pea plants to study heredity in a series of experiments. Mendel worked by carefully observing and recording traits in successive generations of plants. Knowledge about DNA and chromosomes came later.
This lab will apply genetic laws to human inheritance using Punnett squares.
Recall that DNA is wound tightly into chromosomes. Cells with only one set of chromosomes, such as sex cells, are haploid. When two haploid cells fuse during fertilization, a diploid zygote with two full sets of chromosomes is formed. Most cells of a mature individual are diploid.
Homologous chromosomes have the same genes, but they might have different versions (alleles) of those genes. Diploid cells have two alleles for each gene. These alleles might be identical (gene A) or different (gene B). Each gene's locus is its location on a chromosome.
Human traits come through dominant or recessive inheritance. For example, the cystic fibrosis traits carried by a dominant allele are always expressed, even if the recessive gene is present (FF or Ff). The recessive is only expressed when two copies of the recessive allele are present (ff).
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Mother: Healthy carrier
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F
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f
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Father: Healthy carrier
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F
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FF
Healthy non-carrier
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Ff
Healthy carrier
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f
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Ff
Healthy carrier
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FF
Affected
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Human gender is carried on the X and Y chromosomes. Females are XX and males are XY. Heredity traits such as color blindness, which is the inability to distinguish among some colors, are carried on the X chromosome (Xc). The presence of one normal XC will allow normal vision.
In this next set of exercises, you will determine the genotypes of the parents by considering the inheritance patterns of traits in their children. The following is a table of the phenotypes of the family members:
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Phenotype
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Alleles
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Parents
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Mother
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Not color blind
Freckles
Type B blood
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Xc XC
Ff
IBi
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Father
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Color blind
No freckles
Type A blood
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XcY
Ff
IAi
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Children
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Abby
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Color blind
Freckles
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Xc Xc
Ff or ff
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Brady
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Not color blind
No freckles
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XCY
ff
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Carly
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Not color blind
No freckles
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Xc XC
ff
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Dennis
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Color blind
Freckles
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XcY
Ff or ff
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Exercise 1: Color Blindness
Using the alleles XC (not color blind) and Xc (color blind), distribute the gametes from each parent to the outside of the Punnett square. Drag and drop the child with the correct phenotype to the box within the Punnett square that has the corresponding genotype that would occur from the fusion of egg and sperm as indicated by your placement of the gametes.
Exercise 2: Freckles
Freckles are groups of cells on the skin that produce the pigment melanin, often in response to exposure to ultraviolet (UV) light. The gene for freckles is inherited in a dominant/recessive pattern. A person carrying even a single copy of the dominant allele (F) will have freckles. A person who is homozygous recessive (ff) will have no freckles.
Using the alleles F (freckles) and f (no freckles), distribute the gametes from each parent to the outside of the Punnett square. Drag and drop the child with the correct phenotype to the box within the Punnett square that has the corresponding genotype that would occur from the fusion of egg and sperm as indicated by your placement of the gametes.
Exercise 3: Blood Type
In codominance, more than one allele encodes a functional protein. IA codes for A markers on red blood cells. IB codes for B markers. i codes for neither A nor B markers, no markers. Human blood types IAIA and IAi are Type A blood. IBIB and IBi are Type B blood. IAIB is a codominant expression of alleles and results in type AB blood. ii is Type O blood.
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Genotypes
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Phenotypes
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IAIA
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IAi
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Blood Type A
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IBIB
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IBi
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Blood Type B
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IAIB
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Blood Type AB
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ii
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Blood Type O
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These parents have a child with Type O (ii) blood. Investigate the possible genotypes for each parent to determine how this genetic outcome would occur. Why must each parent carry the i allele to have a child with Type O blood? Drag and drop the gametes from each parent to the outside of the Punnett square. Drag and drop the corresponding zygote to each block of the Punnett square.