Honors Bio Ch 7 Notes
Vocabulary
Heterozygous – Has two different alleles for a trait. Ex: Ss
Homozygous – Has two of the same alleles for a trait. Ex: SS or ss
Genotype – the combination of alleles that an individual has for a trait. Ex: Ss
Phenotype – the physical trait that results from a genotype. Ex: Sickle cell anemia
Sex-linked gene – located on the X or Y chromosome. Expressed more frequently in males than
females.
Carrier – heterozygous for a recessive allele for a disorder (ex: Cc for Cystic Fibrosis or
XHYh for Hemophilia). They carry the defective allele so they can pass it on to their
offspring, but they do not express the phenotype.
X chromosome inactivation - female mammals, one of the two X chromosomes is randomly turned
off in each cell
Polygenic trait – traits controlled by multiple genes. Results in a wide range of phenotypes (ex:
eye color)
Epistatic gene – a gene that interferes with the expression of other genes
Codominance – both alleles for a trait are completely expressed. Ex: AB Blood type
Incomplete dominance – the heterozygous genotype is a blend of the two
homozygous phenotypes. Ex: Japanese 4 ‘0 clock flowers RR = red,
WW = white, RW = pink.
Karyotype – picture of all the chromosomes in someone’s cells. Can show some
chromosomal disorders. Autosomes are the 1st 22 pairs of chromosomes, sex
chromosomes are the last pair (#23)
Pedigree – Diagram that traces a particular trait in a family. Can show genotype
and/or phenotype.
Chapter Notes
7.1 Chromosomes and Phenotypes
Autosomal genes – genes located on any autosomal chromosome
 two copies are present (one on each homologous chromosome
 both copies affect phenotype
Recessive Genetic Disorders
 An individual must have 2 recessive alleles to have the disorder (cc)
 Carriers are heterozygous (Cc) so they don’t have the disorder but can pass it on to their
offspring. This allows the allele to “hide out” in a population.
 Cystic fibrosis affects sweat and mucus glands.
Dominant Genetic Disorders
 Less common. An individual can be homozygous dominant (HH) or heterozygous (Hh) to
have the disorder.
 Ex: Huntington’s disease – affects the nervous system and appears in adulthood
Sex–linked Genes – genes located on sex chromosomes
 Males: XY
Females XX
 Genes on the X chromosome are expressed more frequently than in females. Why?
 When will a sex-linked trait be expressed in females?
 X chromosome inactivation – one of two X chromosomes is randomly “turned off”.
Ex: Fur color in cats is a sex-linked trait resulting from several genes. Tortoiseshell
and Calico cats are females that express different genes in different cells across their
bodies.
7.2 Complex Patterns of Inheritance
Incomplete Dominance
 Heterozygous phenotype is between the two homozygous phenotypes.
 Ex: Japanese 4 ‘0 clock flowers RR = red, WW = white, RW = pink.
Codominance
 Both alleles are completely expressed so both phenotypes show up.
 Ex: ABO blood types
Phenotypes: Type A, Type B, Type AB, Type O
Polygenic traits
 Traits produced by 2 or more genes that influence the phenotypes
 Produces a continuous range of phenotypes Ex: Human skin color
 Epistatic gene – a gene that interferes with the expression of other genes. Ex: fur color in
mice is controlled by many genes, but one gene (epistatitic gene) overshadows all the
others and results in albinism.
Environment and gene expression
 Environmental conditions influences the expression of some genes
 Ex: Sea turtles – eggs that mature in warmer temperatures develop into females and eggs
that mature in cooler temperatures develop into male turtles.
7.4 Human Genetics and Pedigrees
Pedigree – Chart for tracing genes in a family
 See Figure 7.14 You should be able to look at a pedigree and identify genotype and
phenotype for given individuals.
 You must first determine if the trait is a result of an autosomal or sex-linked gene.
Karyotype – a picture of the chromosomes in a cell
 Can help diagnose chromosomal disorders
 Loss of part of a chromosome (deletion)
 Down syndrome (trisomy 21)– extra copy of 21st chromosome. This is a result of
nondisjunction (chromosomes fail to separate during meiosis.
How might this result in gametes with missing or extra chromosomes?