3.3 Genetics Study Guide
Sexual Reproduction
 Sorting and recombination of genes during sexual reproduction has an effect on variation in offspring
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Meiosis- formation of four gametes (sperm and eggs) with the haploid number of chromosomes
o 2n  n (diploid  haploid)
o Chromosome number is reduced by half
Fertilization- combination of gametes to form a zygote (fertilized egg)
o n + n  2n
o zygote has diploid number of chromosomes
Autosomes- body cells (all cells except gametes)
Sex chromosomes- reproductive cells
o Gametes (sperm and egg)
o 23rd pair of homologous chromosomes
 Males are XY
 Females are XX
Karyotypes- photograph of cells during mitosis with chromosomes arranged in homologous pairs
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Crossing-over- homologous chromosomes exchange portions of
their chromatids (and exchange alleles)
o Results in new gene combinations and therefore more
variation within species
Genes and Traits
 Expressed traits are passed from parents to offspring
 Genetic traits are determined by the code in a DNA molecule
 Genes- segments of DNA that code for RNA or a protein
 DNA storage
o DNA is protected on chromosomes inside the double-membraned nucleus
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Allele- form of a gene
Dominant traits- expressed when at least one dominant allele is present
Recessive traits- expressed only in the absence of a dominant allele
Phenotypes- expressed physical traits
o Examples:
 Brown fur
 Tall stems
Genotypes
o Represented by pairs of alleles
o Homozygous
 Same alleles
 Examples: BB and bb
o Heterozygous
 Different alleles
 Example: Bb
Translation of genotypes into phenotypes
o Genotypes with at least one dominant allele express the dominant phenotypes
 Example: If B- brown and b- gray, rabbits with the genotypes BB and Bb will express the
dominant phenotype, brown.
o Genotypes with two recessive alleles express the recessive phenotype
 Example: If B- brown and b- gray, rabbits with the genotype bb will express the
recessive phenotype, gray.
Punnett Squares- used to predict and/or interpret the results of a genetic cross
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Ratios, Proportions, and Percentages
o Example: 2 out of 8 offspring
 Ratio is 2:8
 Proportion is ¼
 Percentage is 25%
o Example: 14 out of 30 offspring
 14:30 = ½ = 50%
o Example: 12 out of 16 offspring
 12:16 = ¾ = 75%
o Example: All offspring express the trait
 1:0 = 4/4 = 100%
Translating genotypes into phenotypes
o If B is dominant for brown and b is recessive for white:
 BB and Bb are brown
 Bb is white
Determining parents’ genotypes
o Complete punnett squares for all possible genotypes, analyze results and compare to prompt
Pedigrees and Sex-Linked Traits
 Used to interpret patterns of inheritance within a family
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Symbols
o Males- squares
o Females- circles
o Unshaded- unaffected
o Half-shaded circles- carrier females
o Completely shaded- affected
Sex-linked Traits
 X-linked traits are passed from mothers to sons
 X-linked traits are found on the 23rd X chromosome
o Females have two X chromosomes
 If the trait is recessive, females would have to have 2 recessive alleles to be affected
 If the trait runs in the family, there is a 1/3 chance a female would be affected, 1/3
chance the female is a carrier, and 1/3 chance she would not carry the allele at all
o Males have only one 23rd X chromosome
 If the trait is recessive, males only have to have 1 recessive allele to be affected
 If the trait runs in the family, there is a ½ chance a male would be affected and ½ chance
he would be unaffected (and not even a carrier)
 Determining if a trait is sex-linked from pedigree analysis
o Only females are heterozygous (half-shaded, carriers)
o More males than females are affected
Structure of DNA and RNA
 DNA structure
o Double-stranded helix
o Nucleotide bases are G, C, A, and T
o Sugar-phosphate backbone
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 Sugar is deoxyribose
RNA structure
o Single-stranded
o Sugar-phosphate backbone
 Sugar is ribose
o Nucleotide bases are G, C, A, and U (uracil)
o Forms of RNA
 mRNA- messenger RNA
 encodes amino acid sequences
 tRNA- transfer RNA
 brings amino acids to ribosomes during translation
 rRNA- ribosomal RNA
 makes up the ribosomes along with proteins
Protein Synthesis
 The sequence of nitrogen bases directs protein formation
 DNA  complementary DNA  RNA  amino acid sequence  protein
o Replication- occurs in nucleus
 DNA molecule makes a copy of itself
 Enzyme unzips double helix
 Enzymes base pair exposed bases on each strand
o Base pair rules:
 C and G base pair
 A and T base pair
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A complementary strand of DNA is made and transported out of the nucleus to
the cytoplasm
o Transcription- occurs in cytoplasm
 Complementary DNA used to make RNA
 Base pair rules:
 C and G base pair
 T base pairs with A
 A base pairs with U
o Translation- occurs in cytoplasm
 Three bases in RNA (codons) code for amino acids (follow codon chart on page 303 in
textbook)
 Amino acids are then polymerized by ribosomes to make proteins
o Proteins determine traits!
DNA Alterations
 The effects of DNA alteration can be beneficial and harmful to individuals, society, and the
environment
o Beneficial: allows for variation
o Harmful: results in deformation, disease, and/or death
 Mutations
o An error occurs at some point during protein synthesis
 Example: a change in DNA base sequence results in production of the wrong protein
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Chromosomal abnormalities- identified by karyotype analysis
o Chromosome shape
 Part of the chromosome may be duplicated, deleted, inverted, or translocated
o Chromosome number
 Error during meiosis results in wrong number of chromosomes in gametes and therefore
wrong number in zygote
 Examples: Down Syndrome and Sex chromosome disorders
Genetic engineering
o Gene splicing- removing genes from one organism and inserting them into the DNA of another
o Recombinant DNA- DNA produced by combining DNA from different sources
o Cloning- using one cell to produce cells that are genetically identical