Download 1 Inheritance 1

To Start: Turn to p99-101
1. What is a locus?
2. Why is the fruit fly often used in genetics?
3. Why are certain characteristics more likely to
be inherited together?
4. How did geneticists determine which genes
were closer together?
5. What do we mean by “cross over value”?
6. What do geneticists use as symbols for the
short arm of the chromosome and for the
long?
To Start: Turn to p99-101
1. What is a locus? Position of a gene on a chromosome
2. Why is the fruit fly often used in genetics? Short life cycle,
defined phenotypes, easy to culture, only 4 chromosomes.
3. Why are certain characteristics more likely to be inherited
together? If they are located close together on a
chromosome.
4. How did geneticists determine which genes were closer
together? Carrying out crosses and determining how
frequently genes were separated by crossing over.
5. What do we mean by “cross over value”? High cross over
value – genes are far away, low – genes are close and less
likely to be separated at crossing over.
6. What do geneticists use as symbols for the short arm of the
chromosome and for the long? p – short, q - long
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Genetics Terms
Write the word on one side of the card and
the definition on the other.
Gene
Diploid
Recessive
Homozygous
Zygote
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Allele
Locus
Haploid
Dominant
Genotype
Phenotype
Heterozygous Gamete
Homologous chromosomes
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Key terms
Alleles are different forms of a gene. A gene is a sequence
of bases on a DNA molecule that codes for a polypeptide.
Within a cell, there may be pairs of homologous
chromosomes, which have the same genes at the same
locus. The cell is diploid. The zygote (fertilised egg) and
other somatic (body) cells are diploid.
If a cell contains one chromosome from each homologous
pair, the cell is haploid. The gametes (sex cells) are
haploid.
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Key terms
If the alleles for a characteristic are the same, the organism
is said to be homozygous for that characteristic. The
organism is a homozygote.
If the alleles for a characteristic are different, the organism
is said to be heterozygous for that characteristic. The
organism is a heterozygote.
An allele is dominant if it is always expressed in the
phenotype, even if the organism is heterozygous.
An allele is recessive if it is only expressed in the phenotype
when the organism is homozygous.
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Key terms
The genotype of an organism is determined by the
combination of alleles it has.
The phenotype of an organism is determined by the
genotype and the environment.
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Representing alleles
A gene can be represented using a letter: upper case for the
dominant allele, and lower case for the recessive allele.
For example, the allele for wing length in Drosophila can
be either long (L) or short (l).
Genotype
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Phenotype
LL
homozygous dominant
long wings
Ll
heterozygous
long wings
ll
homozygous recessive short wings
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Task
Working in pairs:
- 1 person places their cards with the
word facing up
- 1 person places their cards with the
definition facing up
Time each other to see how quickly you can
match the words to the correct definitions.
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