Genetics and Heredity
Dominant and Recessive Traits
Learning Goal:
I can explain the difference
between dominant and recessive
traits
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Human Genetics
• Genes code for specific traits i.e. eye colour, hairline, earlobes, ,
freckles, dimples, blood type, etc.
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What’s going on here?
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Take a minute to think about what you already know in regards to this picture……………..
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Human Genetics
*Workbook Notes*
• Each chromosome contains sections of DNA called genes,
that code for specific traits.
*Draw the diagram*
• Each gene is on a particular chromosome
at a specific, fixed location called a locus
(plural loci)
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• Chromosomes are made up of the
DNA molecule and specific areas
along the DNA molecule contain the
genes.
• Genes are often separated by noncoding (“junk”) DNA, the function of
which is not yet known.
• Different genes are responsible for
different characteristics
e.g. eye colour, height, skin colour,
hair colour and shape.
• We have seen how chromosomes can be sorted into
matching pairs.
- The genes at the same location on a pair of
chromosomes affect the same characteristic (trait).
For example,
This matching pair of chromosomes
shows the gene that controls your
earlobe.
Notice that although that each gene
controls the same feature their effect
may be different.
Human Genetics - Alleles
*Workbook Notes*
• Genes can have different versions known as alleles
*Draw the diagram*
Genotype v Phenotype
• The combination of alleles you have is
known as your genotype; that is, what is
actually in your DNA.
• The traits that are expressed (show up) are
known as your phenotype; that is, what
you might physically see, e.g. freckles.
• Phenotypes can also be influenced by the
environment.
•
Hint: ‘Geno’ = Genes
‘Pheno’ = Physical characteristic
Dominant and recessive alleles
• Alleles can be dominant or recessive.
• The possession of one or two dominant
alleles will show up in the phenotype as
the dominant trait,
• however two copies of the recessive
allele are needed for the recessive trait to
be shown
Dominant and recessive alleles
• While genes usually have a scientific name e.g. BRCA1 (breast cancer 1), for
simple genetics, genes are given a letter
• The dominant allele, hence the dominant trait, is allocated a CAPITAL letter
• The recessive allele is allocated a lower case letter
Dominant and recessive alleles
• For example, in the gene for hair colour, black hair is dominant to
red/blonde hair
• So the dominant allele would be B and the recessive allele would be b
• Therefore, the possible genotypes for hair
colour could be BB, Bb or bb
Dominant and recessive alleles
• Remember we have 2 copies of each gene (one from mum and one
from dad)
• So an individual with black hair could have the genotype BB or Bb;
their phenotype is black hair
• An individual with red hair would have the genotype bb and their
phenotype is red hair
So could this lady be
this kid’s mum??
Watch this clip and then think about your answer. https://www.youtube.com/watch?v=pv3Kj0UjiLE
Dominant and recessive traits
• 1. Either read the next three slides, or download the
‘Chromosomes and genes’ and ‘More about chromosomes’
worksheets from Compass Resources.
• 2. Answer ‘Chromosomes and genes’ Questions 1 to 5 in your
workbook
• 3. Answer ‘More about chromosomes’ Qu 1 to 3 in your
workbook
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• a. EE – brown
• b. Ee – brown
• c. ee - blue
• Heterozygous: Ee
• Homozygous: EE and ee