Where new gene combinations come from
Grade 9 Science Worksheet
Circle the best answer.
“Chromosome cards (model, not to scale). One cell with 2 sets of chromosomes; pair 1 is shown. Before the crossover,” Ari follows a pair with genes A and B. The dark chromosome has A B and the light one has a b. One crossover happens between the genes. Which list shows the four chromatids afterward?
- A B, A B, a b, a b
- A b, A b, a B, a B
- A B, A b, a B, a b
- A B, A b, a b, a b
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Ten model cells with one crossover each
item value cells 10 crossovers in each cell 1 chromatids in each cell 4 Bo models 10 cells. In each cell one crossover swaps pieces between one dark and one light chromatid of a pair. How many chromatids in the 10 cells are recombinant?
- 12 chromatids
- 10 chromatids
- 20 chromatids
- 40 chromatids
Use this
Chromosome cards (model, not to scale). One cell with 2 sets of chromosomes; each chromosome has 2 chromatids. Pair 1: a dark chromosome with alleles A and B and a light chromosome with alleles a and b. Pair 2: a dark chromosome with allele C and a light chromosome with allele c. The cell is early in meiosis, with each pair lying side by side.
Esme looks at a cell with 2 pairs of chromosomes. Genes A and B are on pair 1, and gene C is on pair 2. Which chromatids can swap pieces in crossing over?
- The two sister chromatids of one chromosome, in prophase I
- A dark chromatid of pair 1 and a light chromatid of pair 2, in prophase I
- A dark and a light chromatid of pair 1, in prophase II
- A dark and a light chromatid of pair 1, in prophase I
Pea plant fact card
item value pairs of chromosomes 7 ways each pair can line up 2 Jun reads that a pea plant has 7 pairs of chromosomes. How many line-ups can the 7 pairs make at the middle, before crossing over?
- 49 line-ups
- 2 line-ups
- 128 line-ups
- 14 line-ups
Model cell card
item value line-ups at the middle 64 ways each pair can line up 2 A model cell can line up its chromosomes in 64 different ways at the middle. Kira wants the number of pairs. How many pairs does it have?
- 64 pairs
- 6 pairs
- 32 pairs
- 8 pairs
Human cell fact card
item value pairs of chromosomes 23 ways each pair can line up 2 Luz reads that human cells have 23 pairs of chromosomes. About how many line-ups can the 23 pairs make at the middle, before crossing over?
- 46 line-ups
- 2 line-ups
- About 8.4 million line-ups
- 529 line-ups
Two parents (model data)
item value pairs in each parent 3 kinds of gametes from each parent 8 Milo reads a card for two parents. Each can make 8 kinds of gametes by independent assortment alone. How many offspring combinations can random fertilization give?
- 16 combinations
- 8 combinations
- 64 combinations
- 24 combinations
Two human parents (arithmetic)
item value pairs in each parent 23 line-ups for each parent 8,388,608 line-ups for the two parents together ? Pia reads the arithmetic for two parents with 23 pairs each. Which statement gives the count for combined line-ups after random fertilization?
- 2²³ + 2²³ = 2²⁴, about 16.8 million, since the two parents' line-ups form one longer list
- 2²³ × 2²³ = 2⁴⁶, about 70 trillion, since any line-up from one parent can meet any from the other
- 1, since each parent passes on one fixed set of chromosomes to the offspring
- 2²³, about 8.4 million, since just one parent's line-ups count for the offspring
Chromatid card (model)
item value top allele on this chromatid H bottom allele on this chromatid j alleles on the dark chromosome H and J alleles on the light chromosome h and j Tala finds a chromatid with allele H from the dark chromosome and allele j from the light one. Which process made it, and what does that process make?
- Crossing over: chromatids that mix both homologs, in prophase II
- Crossing over: chromatids with brand-new alleles, in prophase I
- Crossing over: chromatids that mix both homologs, in prophase I
- Independent assortment: whole chromosomes in new mixes, in metaphase I
Four sources of variation (model)
source what it does crossing over swaps pieces between non-sister chromatids independent assortment lines up pairs at random random fertilization joins one gamete with another mutation changes the DNA sequence of a gene A model plant has a flower-color allele that neither parent had. Which source of variation is the ultimate source of new alleles, asks Zee?
- Random fertilization, because joining cells build new alleles
- Independent assortment, because pairs mix into new alleles
- Mutation, a change in the DNA sequence of a gene
- Crossing over, because swapped pieces form new alleles