The document discusses several key concepts relating to inheritance of genes from parents to offspring, including loci, alleles, genotypes, phenotypes, dominance, recessivity, codominance, monohybrid and dihybrid crosses, sex linkage, multiple alleles, and test crosses. It provides examples and diagrams to illustrate genetic crosses and how they are used to determine the genotypes and inheritance of traits.

1. Inherited Change and
Gene Technology
The passage of information
from parent to offspring
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2. • Locus – position on the chromosome
in which the gene resides
*plural: loci
• Alleles – a particular variety of the
same gene
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3. • Homozygous – 2 identical alleles of a
gene
• Heterozygous – 2 different alleles of a
gene
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4. • Genotype – alleles possesed by an
organism
• Phenotype – observable
characteristics/physical traits of an
organism (resulting from interaction
between genotype and environment)
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5. • Dominant – when effect on the phenotype of a
heterozygote is identical to its effect in a
homozygote
• Recessive – only expressed when no dominant
allele is present
• Codominant – both alleles have an effect on
the phenotype of a heterozygous organism
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6. Genetic Diagram (monohybrid
crosses)
• Let N be the dominant allele for normal pigment.
• Let n be the recessive allele for albino.
Parent phenotype Heterozygous Normal Heterozygous
Normal
Parent genotype Nn X
Nn
Gametes N n
N n
Possible offspring
genotype NN Nn Nn
nn
Offspring phenotype Normal Normal Normal
Albino
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7. Genotypes of parent
N n
NN Nn
N
Genotypes
Normal Normal
of parent
Nn nn
n
Normal Albino
Ratio of phenotype 3 normal : 1 albino
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8. Genetic Diagram (dihybrid crosses)
• Let A be the dominant allele for purple stem and a
recessive green
• Let D be the dominant allele for cut leaves and d
recessive smooth
Parent phenotype Heterozygous Heterozygous
Parent genotype AaDd X AaDd
Gametes AD Ad aD
ad
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9. Genotypes of parent
AD Ad aD ad
AADD AADd AaDD AaDd
AD Purple, Purple, Purple, Purple,
cut cut cut cut
AADd AAdd AaDd Aadd
Ad
Purple, Purple, Purple, Purple,
Genotype cut smooth cut potato
s of
AaDD AaDd aaDD aaDd
parent
aD Purple, Purple, Green, Green,
cut cut cut Cut
AaDd Aadd aaDd aadd
ad Purple, Purple, Green, Green,
cut smooth Cut smooth
Ratio of phenotype 9 purple,cut:3 purple,smooth:3 green,cut:1 green,smooth
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10. • Sex linkage – related to genes that are
carried on an X chromosome but not on a
Y chromosome
• Codominance ?
• Multiple alleles – the existence of 3 or
more alleles of a gene (eg. determination
of A, B, and O blood groups)
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11. Test Crosses
• Involves crossing an organism showing
the dominant phenotype with one which is
homozygous recessive to find out if the
dominant genotype is hetero- or
homozygous.
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