Chapter 2
From Conception to Birth
Genes and Chromosomes
● Conception begins as a zygote, single cell formed from the union of two
gametes: sperm and ovum
○ Homozygous: referring to two genes of one pair that are exactly the same
in every letter in their code. Most gene pairs are homozygous
● DNA (deoxyribonucleic acid): the chemical composition of the molecules that
contain the genes, which are the chemical instructions for cells to
manufacture various proteins
● Chromosome: one of the 46 molecules of DNA that virtually every cell of the
human body contains and that contain all of the genes. Other creatures has
fewer or more chromosomes
○ Humans have 21,000 genes
○ 46 chromosomes, which are arranged in 23 pairs
◆ not in a gamete (otherwise called a reproductive chromosome)
Variations among people
● Allele: a variation that makes a gene different in some way form other genes
for the same characteristics. Many genes never vary; others have several
possible alleles
○ Heterozygous: referring to two genes of one pair that differ in some way.
Typically one allele has only a few base pairs that differ from the other
member of the pair
Triplets: set of three three pairs where each specifies production of one of 20
possible amino acids
Genetic expression can be important whether or not a gene is expressed, which
means that it becomes active in forming the person
– No single genes for homosexuality, just combinations of it
Myelination alters genes and continues throughout life
● Microbiome: all of the microbes with all of their genes in a community
● Copy number variations: the various repeats of deletions of base pairs that
genes have
○ Everyone has them, correlates with every disease and conditions
Genotype and Phenotype
● Genotype: a collection of genes within a person
● Phenotype: person's actual appearance and behavior; observable
characteristics of a person
, Shared and Divergent Genes
● Genome: an entire set of genes to make a member of a certain species
○ Every race share 99.5% of their genetic codes
○ Humans and chimpanzee are 98% same
Male and Female
● XX female
● XY male
○ 120 males for every 100 females (120:100) probably b/c smaller Y sperm
can swim faster than heavier X sperm and thus reach he ovum first
Y chromosome has fewer genes than X, but has one crucial gene: SRY which
directs the developing fetus to make male rogans
● Autosomes are inherited by males or females (22 of 23)
Twins
● zygote splits completely producing genetically identical cells
● Monozygotic (MZ) twins: twins from one zygote that splits apart very early in
development. Other monozygotic multiple births such a s triplets can occur
○ don't have the same phenotype, develop their own identities (nurture)
● Dizygotic twins (fraternal): twins who are formed when tow separate ova are
fertilized by two separate sperm at roughly the same time. Only have half their
genes in common
Genetic Interactions
● Additive genes: the effects add up and make the phenotype (height, hair, skin
color); genes that each contribute to the characteristics
● Dominant-recessive pattern: when alleles interact and the dominant gene is
more influential than the recessive gene (nonaddictive)
○ if you inherit the recessive gene not present in the phenotype you are a
carrier of that gene
● Carrier: a person whose genotype includes a gene that is not expressed in the
phenotype. The carried gene occurs in half the carrier’s gametes and thus is
passed on to half of the carrier’s children
○ If inherited from both parents, the characteristic appears on phenotype
● X-linked: a gene carried on the X chromosome. If male inherits an X-linked
recessive trait, he expresses that trait b/c the Y from his father has no
counteracting gene
○ Can be cause of colorblindness
○ 20x more boys than girls
The final complexity mentioned here is epigenetic. Genes are affected form the
moment of conception by other materials.
– All important human characteristics are epigenetic including diseases and
disabilities
From Conception to Birth
Genes and Chromosomes
● Conception begins as a zygote, single cell formed from the union of two
gametes: sperm and ovum
○ Homozygous: referring to two genes of one pair that are exactly the same
in every letter in their code. Most gene pairs are homozygous
● DNA (deoxyribonucleic acid): the chemical composition of the molecules that
contain the genes, which are the chemical instructions for cells to
manufacture various proteins
● Chromosome: one of the 46 molecules of DNA that virtually every cell of the
human body contains and that contain all of the genes. Other creatures has
fewer or more chromosomes
○ Humans have 21,000 genes
○ 46 chromosomes, which are arranged in 23 pairs
◆ not in a gamete (otherwise called a reproductive chromosome)
Variations among people
● Allele: a variation that makes a gene different in some way form other genes
for the same characteristics. Many genes never vary; others have several
possible alleles
○ Heterozygous: referring to two genes of one pair that differ in some way.
Typically one allele has only a few base pairs that differ from the other
member of the pair
Triplets: set of three three pairs where each specifies production of one of 20
possible amino acids
Genetic expression can be important whether or not a gene is expressed, which
means that it becomes active in forming the person
– No single genes for homosexuality, just combinations of it
Myelination alters genes and continues throughout life
● Microbiome: all of the microbes with all of their genes in a community
● Copy number variations: the various repeats of deletions of base pairs that
genes have
○ Everyone has them, correlates with every disease and conditions
Genotype and Phenotype
● Genotype: a collection of genes within a person
● Phenotype: person's actual appearance and behavior; observable
characteristics of a person
, Shared and Divergent Genes
● Genome: an entire set of genes to make a member of a certain species
○ Every race share 99.5% of their genetic codes
○ Humans and chimpanzee are 98% same
Male and Female
● XX female
● XY male
○ 120 males for every 100 females (120:100) probably b/c smaller Y sperm
can swim faster than heavier X sperm and thus reach he ovum first
Y chromosome has fewer genes than X, but has one crucial gene: SRY which
directs the developing fetus to make male rogans
● Autosomes are inherited by males or females (22 of 23)
Twins
● zygote splits completely producing genetically identical cells
● Monozygotic (MZ) twins: twins from one zygote that splits apart very early in
development. Other monozygotic multiple births such a s triplets can occur
○ don't have the same phenotype, develop their own identities (nurture)
● Dizygotic twins (fraternal): twins who are formed when tow separate ova are
fertilized by two separate sperm at roughly the same time. Only have half their
genes in common
Genetic Interactions
● Additive genes: the effects add up and make the phenotype (height, hair, skin
color); genes that each contribute to the characteristics
● Dominant-recessive pattern: when alleles interact and the dominant gene is
more influential than the recessive gene (nonaddictive)
○ if you inherit the recessive gene not present in the phenotype you are a
carrier of that gene
● Carrier: a person whose genotype includes a gene that is not expressed in the
phenotype. The carried gene occurs in half the carrier’s gametes and thus is
passed on to half of the carrier’s children
○ If inherited from both parents, the characteristic appears on phenotype
● X-linked: a gene carried on the X chromosome. If male inherits an X-linked
recessive trait, he expresses that trait b/c the Y from his father has no
counteracting gene
○ Can be cause of colorblindness
○ 20x more boys than girls
The final complexity mentioned here is epigenetic. Genes are affected form the
moment of conception by other materials.
– All important human characteristics are epigenetic including diseases and
disabilities
